Employee Benefit Plans (Narrative) (Details)	12 Months Ended
Dec. 31, 2019 USD ($) decimal shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Minimum Years Of Service For Eligible Employees Of Postretirement Plans	1 year
Minimum Number Of Hours Worked By Employees \| decimal	1,000
Automatic Cash Contribution	2.00%	2.00%	2.00%
Defined Contribution Plan, Employer Discretionary Contribution Amount	$ 17,000,000	$ 18,000,000	$ 18,000,000
Defined Contribution Plan, Employer Matching Contribution, Percent	5.00%	4.00%
Defined Contribution Plan, Cost	$ 58,000,000	$ 48,000,000	48,000,000
Total Company Common Stock Shares Held Under Defined Contribution Plan \| shares	21,000,000	23,000,000
Dividends Earned By Defined Contribution Plan	$ 13,000,000	$ 10,000,000	$ 8,000,000
Qualified Plan [Member] \| Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Benefit Obligation	$ 2,100,000,000	1,800,000,000
Expected long-term rate of return on plan assets for net periodic benefit cost, next fiscal year	6.74%
Number of shares held in plan assets relating to company's common stock (whole number) \| shares	2,855,618
Market Value Of Companys Common Stock Held In Plan Assets	$ 49,000,000.0
Approximate percentage of company's common stock shares held in plan assets	2.20%
Qualified Plan [Member] \| Equity Securities [Member] \| Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation For Defined Benefit Plan Equity Securities	51.00%
Qualified Plan [Member] \| Fixed Income Securities [Member] \| Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation For Defined Benefit Plan Equity Securities	37.00%
Qualified Plan [Member] \| Other Security Investments [Member] \| Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation For Defined Benefit Plan Equity Securities	12.00%
Nonqualified Plan [Member] \| Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Benefit Obligation	$ 171,000,000	$ 141,000,000

2% cash 401(k) Contribution to eligible employees regardless of whether or not they were contributing to the 401(k) plan. To receive the contribution, employees must not be actively accruing a benefit in the Region's pension plan and must be employed at the end of the year

No definition available.