Benefit Plans - Fair Value of pension plan assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
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)
Free Standing Derivative Liabilities | Significant other observable inputs (Level 2)
Components of net periodic cost
Total plan assets at fair value		(17)
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	$ 3,122	$ 3,020
Total plan assets at fair value, allocation (as a percent)	100.00%	100.00%
Investments measured using the net asset value practical expedient	$ 3,908	$ 3,418
Securities lending obligation	(101)	(272)
Derivatives counterparty and cash collateral netting	(19)	9
Other net plan assets	77	17
Fair value of plan assets	6,987	6,192	$ 5,299
Pension benefits | Quoted prices in active markets for identical assets (Level 1)
Components of net periodic cost
Total plan assets at fair value	$ 469	$ 579
Total plan assets at fair value, allocation (as a percent)	15.00%	19.20%
Pension benefits | Significant other observable inputs (Level 2)
Components of net periodic cost
Total plan assets at fair value	$ 2,651	$ 2,441
Total plan assets at fair value, allocation (as a percent)	84.90%	80.80%
Pension benefits | Significant unobservable inputs (Level 3)
Components of net periodic cost
Total plan assets at fair value	$ 2	$ 0
Total plan assets at fair value, allocation (as a percent)	0.10%	0.00%
Fair value of plan assets	$ 2	$ 0	5	$ 39
Pension benefits | Equity securities
Components of net periodic cost
Total plan assets at fair value	269	261
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	227	216
Pension benefits | Equity securities | Significant other observable inputs (Level 2)
Components of net periodic cost
Total plan assets at fair value	42	45
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	29
Pension benefits | U.S. government and agencies
Components of net periodic cost
Total plan assets at fair value	897	1,333
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	32	237
Pension benefits | U.S. government and agencies | Significant other observable inputs (Level 2)
Components of net periodic cost
Total plan assets at fair value	865	1,096
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	1,711	1,060
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	1,709	1,060
Pension benefits | Corporate | Significant unobservable inputs (Level 3)
Components of net periodic cost
Total plan assets at fair value	2	0
Fair value of plan assets	2	0	$ 5	$ 10
Pension benefits | Short-term investments
Components of net periodic cost
Total plan assets at fair value	245	380
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	210	128
Pension benefits | Short-term investments | Significant other observable inputs (Level 2)
Components of net periodic cost
Total plan assets at fair value	35	252
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	21	5
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	21	5
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	(23)	$ (19)
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	(21)
Pension benefits | Free Standing Derivative Liabilities | Significant unobservable inputs (Level 3)
Components of net periodic cost
Total plan assets at fair value	0
Pension benefits | Free Standing Derivative Other Assets
Components of net periodic cost
Total plan assets at fair value	2
Pension benefits | Free Standing Derivative Other Assets | 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 Other Assets | Significant other observable inputs (Level 2)
Components of net periodic cost
Total plan assets at fair value	0
Pension benefits | Free Standing Derivative Other Assets | Significant unobservable inputs (Level 3)
Components of net periodic cost
Total plan assets at fair value	$ 0