v3.19.3.a.u2
Debt Narratives (Details) - USD ($)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Debt (Textual) [Abstract]      
Maturities of long-term borrowings in 2020 $ 105,000,000    
Maturities of long-term borrowings in 2021 235,000,000    
Maturities of long-term borrowings in 2022 940,000,000    
Maturities of long-term borrowings in 2023 198,000,000    
Maturities of long-term borrowings in 2024 548,000,000    
Short-term Debt [Line Items]      
Letters of Credit Outstanding, Amount 277,000,000    
Debt Instrument [Line Items]      
Debt Instrument, Face Amount 13,971,000,000 $ 13,971,000,000  
Repayment of Debt 80,000,000 4,995,000,000 $ 7,876,000,000
Revolving Credit Facility [Member]      
Short-term Debt [Line Items]      
Total amount under revolving credit facilities 6,000,000,000.0    
Commercial paper program $ 6,000,000,000.0    
Maturity period of commercial paper (in days) P90D    
Minimum limit of debt for cross default provision $ 200,000,000    
Letters of Credit Outstanding, Amount 0 0  
Commercial paper outstanding 0 0  
Line of Credit Facility, Remaining Borrowing Capacity 6,000,000,000.0    
Letter Of Credit [Member]      
Short-term Debt [Line Items]      
Letters of credit capacity 500,000,000    
Unsecured Debt (Debentures) [Member] | Due 2021 [Member] | 9.125% Debentures [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Face Amount 123,000,000 123,000,000  
Unsecured Debt (Debentures) [Member] | Due 2022 [Member] | 2.4% Notes [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Face Amount 329,000,000 329,000,000  
Unsecured Debt (Debentures) [Member] | Due 2023 [Member] | 7.65% Debentures [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Face Amount 78,000,000 78,000,000  
Unsecured Debt (Debentures) [Member] | Due 2024 [Member] | 3.35% Notes [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Face Amount 426,000,000 426,000,000  
Unsecured Debt (Debentures) [Member] | Due 2025 [Member] | 3.35% Notes [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Face Amount 199,000,000 199,000,000  
Unsecured Debt (Debentures) [Member] | Due 2025 [Member] | 8.20% Debentures [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Face Amount 134,000,000 134,000,000  
Unsecured Debt (Debentures) [Member] | Due 2027 [Member] | 7.8% Debentures [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Face Amount 203,000,000 203,000,000  
Unsecured Debt (Debentures) [Member] | Due 2030 [Member] | 8.125% Notes [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Face Amount 390,000,000 390,000,000  
Unsecured Debt (Debentures) [Member] | Due 2034 [Member] | 4.15% Notes [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Face Amount 246,000,000 246,000,000  
Unsecured Debt (Debentures) [Member] | Due 2047 [Member] | 7.9% Debentures [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Face Amount 60,000,000 60,000,000  
Floating Rate Notes [Member] | Due 2018 [Member] | Floating rate note due 2018 [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Face Amount 500,000,000 500,000,000  
Variable rate debt bonds [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Face Amount $ 283,000,000 $ 283,000,000