v3.19.3.a.u2
Pension And Postretirement Benefits (Narrative) (Details) - USD ($)
 3 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Sep. 30, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]                  
Defined Benefit Measurement Date             Dec. 31, 2019    
Preferred equity interest in Mobility               $ 8,803,000,000  
Combined net pension and postretirement cost             $ 2,762,000,000 (4,251,000,000) $ 155,000,000
Increase (decrease) in plan benefit obligations due to change in assumed rates             $ (2,002,000,000) $ (1,148,000,000) (1,031,000,000)
Composite rate of compensation increase for determining benefit obligation 3.00%           3.00% 3.00%  
Pension Contribution Date             Dec. 31, 2019    
Pension Contributions             $ 200,000,000    
Benefit cost of the contributory savings plans             793,000,000 $ 724,000,000 703,000,000
Debt-financed shares held by ESOPs (allocated or unallocated) $ 0           0    
Forecast [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Increase (decrease) in plan benefit obligations due to change in assumed rates           $ 273,000,000      
Percentage decrease in the actual long-term rate of return used to report the impact of change on future combined net pension and postretirement cost           (0.50%)      
Pension Benefit [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Special termination benefits             81,000,000 1,000,000  
Accumulated benefit obligation for pension plans $ 58,150,000,000           58,150,000,000 53,963,000,000  
Combined net pension and postretirement cost             $ 2,393,000,000 $ (909,000,000) $ 651,000,000
Discount rate for determining projected benefit obligation 3.40%           3.40% 4.50% 3.80%
Weighted-average interest crediting rate for cash balance 3.30%           3.30% 3.70% 3.50%
Increase (decrease) in plan benefit obligations due to change in assumed rates             $ 8,018,000,000 $ (4,394,000,000)  
Increase (decrease) in discount rate over prior year             (1.10%) 0.70%  
Long-term rate of return on plan assets             7.00% 7.00% 7.75%
Composite rate of compensation increase for determining benefit obligation 3.00%           3.00% 3.00% 3.00%
Pension Benefit [Member] | Forecast [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Increase (decrease) in plan benefit obligations due to change in assumed rates           $ 130,000,000      
Long-term rate of return on plan assets           7.00%      
Percentage decrease in the actual long-term rate of return used to report the impact of change on future combined net pension and postretirement cost           50.00%      
Pension Benefit [Member] | Mortality Assumption Rate Change [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Increase (decrease) in plan benefit obligations due to change in assumed rates             $ (147,000,000) $ (488,000,000)  
Pension Benefit [Member] | Service Cost [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Discount rate in effect for determining net cost 3.50% 3.90% 4.30% 4.60%     4.10% [1],[2] 4.20% [1],[2] 4.60% [1],[2]
Pension Benefit [Member] | Interest Cost [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Discount rate in effect for determining net cost 3.00% 3.20% 3.70% 4.20%     3.50% [1],[2] 3.80% [1],[2] 3.60% [1],[2]
Postretirement Benefit [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Special termination benefits             $ 0 $ 1,000,000  
Combined net pension and postretirement cost             $ 369,000,000 $ (3,342,000,000) $ (496,000,000)
Discount rate for determining projected benefit obligation 3.20%           3.20% 4.40% 3.70%
Weighted-average interest crediting rate for cash balance 0.00%           0.00% 0.00% 0.00%
Increase (decrease) in plan benefit obligations due to change in assumed rates             $ 2,399,000,000 $ (1,509,000,000)  
Increase (decrease) in discount rate over prior year             (1.20%) 0.70%  
Long-term rate of return on plan assets             5.75% 5.75% 5.75%
Composite rate of compensation increase for determining benefit obligation 3.00%           3.00% 3.00% 3.00%
Postretirement Benefit [Member] | Forecast [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Long-term rate of return on plan assets           4.75%      
Postretirement Benefit [Member] | Mortality Assumption Rate Change [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Increase (decrease) in plan benefit obligations due to change in assumed rates             $ (4,000,000) $ (61,000,000)  
Postretirement Benefit [Member] | Service Cost [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Discount rate in effect for determining net cost 3.40%       4.70%   4.40% [1],[2] 4.30% [1],[2] 4.60% [1],[2]
Postretirement Benefit [Member] | Interest Cost [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Discount rate in effect for determining net cost 2.70%       4.00%   3.70% [1],[2] 3.60% [1],[2] 3.40% [1],[2]
Supplemental Retirement Plans [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Combined net pension and postretirement cost             $ 438,000,000 $ 53,000,000  
Composite rate of compensation increase for determining benefit obligation 3.20%           3.20% 4.40%  
Accumulated benefit obligation $ 2,605,000,000           $ 2,605,000,000 $ 2,397,000,000  
Deferred compensation expense             $ 199,000,000 $ 128,000,000 $ 138,000,000
Administrative Expense [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Estimated annual health care cost trend rate for prior and current year             2.50%    
Administrative Expense [Member] | Forecast [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Estimated annual health care cost trend rate for prior and current year           2.50%      
Dental Claims [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Estimated annual health care cost trend rate for prior and current year             3.00%    
Dental Claims [Member] | Forecast [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Estimated annual health care cost trend rate for prior and current year           3.00%      
Prescription Drug Cost - Eligible Participants [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Estimated annual growth rate for health care cost 4.50%           4.50%    
Prescription Drug Cost - Eligible Participants [Member] | Forecast [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Increase (decrease) in plan benefit obligations due to change in assumed rates           $ (102,000,000)      
Estimated annual growth rate for health care cost           4.00%      
Medical Cost - Eligible Participants [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Estimated annual growth rate for health care cost 4.50%           4.50%    
Medical Cost - Eligible Participants [Member] | Forecast [Member]                  
Defined Benefit Plan Disclosure [Line Items]                  
Increase (decrease) in plan benefit obligations due to change in assumed rates           $ (102,000,000)      
Estimated annual growth rate for health care cost           4.00%      
[1]
Weighted-average discount rate for pension benefits in effect from January 1, 2019 through March 31, 2019 was 4.60% for service cost and 4.20% for interest cost, from April 1, 2019 through June 30, 2019 was 4.30% for service cost and 3.70% for interest cost, from July 1, 2019
[2]
Weighted-average discount rate for postretirement benefits in effect from January 1, 2019 through October 1, 2019 was 4.70% for service cost and 4.00% for interest cost, and, from October 2, 2019 through December 31, 2019 was 3.40% for service cost and 2.70% for interest cost.