Benefit Plans - Fair Value of pension plan assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Free Standing Derivative Liabilities | Quoted prices in active markets for identical assets (Level 1)
Components of net periodic cost
Total plan assets at fair value		$ 0
Free Standing Derivative Liabilities | Significant other observable inputs (Level 2)
Components of net periodic cost
Total plan assets at fair value		(11)
Free Standing Derivative Liabilities | Significant unobservable inputs (Level 3)
Components of net periodic cost
Total plan assets at fair value		0
Pension benefits
Components of net periodic cost
Total plan assets at fair value	$ 3,020	$ 2,809
Total plan assets at fair value, allocation (as a percent)	100.00%	100.00%
Investments measured using the net asset value practical expedient	$ 3,418	$ 2,687
Securities lending obligation	(272)	(222)
Derivatives counterparty and cash collateral netting	9	(6)
Other net plan assets	17	31
Fair value of plan assets	6,192	5,299	$ 6,284
Pension benefits | Quoted prices in active markets for identical assets (Level 1)
Components of net periodic cost
Total plan assets at fair value	$ 579	$ 345
Total plan assets at fair value, allocation (as a percent)	19.20%	12.30%
Pension benefits | Significant other observable inputs (Level 2)
Components of net periodic cost
Total plan assets at fair value	$ 2,441	$ 2,459
Total plan assets at fair value, allocation (as a percent)	80.80%	87.50%
Pension benefits | Significant unobservable inputs (Level 3)
Components of net periodic cost
Total plan assets at fair value	$ 0	$ 5
Total plan assets at fair value, allocation (as a percent)	0.00%	0.20%
Fair value of plan assets	$ 0	$ 5	39	$ 10
Pension benefits | Equity securities
Components of net periodic cost
Total plan assets at fair value	261	316
Pension benefits | Equity securities | Quoted prices in active markets for identical assets (Level 1)
Components of net periodic cost
Total plan assets at fair value	216	51
Pension benefits | Equity securities | Significant other observable inputs (Level 2)
Components of net periodic cost
Total plan assets at fair value	45	265
Pension benefits | Equity securities | Significant unobservable inputs (Level 3)
Components of net periodic cost
Total plan assets at fair value	0	0
Fair value of plan assets	0	0	29	0
Pension benefits | U.S. government and agencies
Components of net periodic cost
Total plan assets at fair value	1,333	681
Pension benefits | U.S. government and agencies | Quoted prices in active markets for identical assets (Level 1)
Components of net periodic cost
Total plan assets at fair value	237	172
Pension benefits | U.S. government and agencies | Significant other observable inputs (Level 2)
Components of net periodic cost
Total plan assets at fair value	1,096	509
Pension benefits | U.S. government and agencies | Significant unobservable inputs (Level 3)
Components of net periodic cost
Total plan assets at fair value	0	0
Pension benefits | Corporate
Components of net periodic cost
Total plan assets at fair value	1,060	1,484
Pension benefits | Corporate | Quoted prices in active markets for identical assets (Level 1)
Components of net periodic cost
Total plan assets at fair value	0	0
Pension benefits | Corporate | Significant other observable inputs (Level 2)
Components of net periodic cost
Total plan assets at fair value	1,060	1,479
Pension benefits | Corporate | Significant unobservable inputs (Level 3)
Components of net periodic cost
Total plan assets at fair value	0	5
Fair value of plan assets	0	5	$ 10	$ 10
Pension benefits | Short-term investments
Components of net periodic cost
Total plan assets at fair value	380	320
Pension benefits | Short-term investments | Quoted prices in active markets for identical assets (Level 1)
Components of net periodic cost
Total plan assets at fair value	128	122
Pension benefits | Short-term investments | Significant other observable inputs (Level 2)
Components of net periodic cost
Total plan assets at fair value	252	198
Pension benefits | Short-term investments | Significant unobservable inputs (Level 3)
Components of net periodic cost
Total plan assets at fair value	0	0
Pension benefits | Free Standing Derivative Assets
Components of net periodic cost
Total plan assets at fair value	5	19
Pension benefits | Free Standing Derivative Assets | Quoted prices in active markets for identical assets (Level 1)
Components of net periodic cost
Total plan assets at fair value	0	0
Pension benefits | Free Standing Derivative Assets | Significant other observable inputs (Level 2)
Components of net periodic cost
Total plan assets at fair value	5	19
Pension benefits | Free Standing Derivative Assets | Significant unobservable inputs (Level 3)
Components of net periodic cost
Total plan assets at fair value	0	0
Pension benefits | Free Standing Derivative Liabilities
Components of net periodic cost
Total plan assets at fair value	(19)	$ (11)
Pension benefits | Free Standing Derivative Liabilities | Quoted prices in active markets for identical assets (Level 1)
Components of net periodic cost
Total plan assets at fair value	(2)
Pension benefits | Free Standing Derivative Liabilities | Significant other observable inputs (Level 2)
Components of net periodic cost
Total plan assets at fair value	(17)
Pension benefits | Free Standing Derivative Liabilities | Significant unobservable inputs (Level 3)
Components of net periodic cost
Total plan assets at fair value	$ 0