Employee Benefits - Change in Plan Assets Measured at Fair Value (Details) - Combined Pension Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Change in plan assets
Fair value of plan assets at beginning of year	$ 10,493	$ 10,033
Fair value of plan assets at end of year	10,546	10,493
Fair Value, Inputs, Level 3
Change in plan assets
Fair value of plan assets at beginning of year	22	24
Acquisitions (dispositions)	(16)	(1)
Actual return on plan assets	2	(1)
Fair value of plan assets at end of year	8	22
High Yield Bonds | Fair Value, Inputs, Level 3
Change in plan assets
Fair value of plan assets at beginning of year	5	7
Acquisitions (dispositions)	1	(2)
Actual return on plan assets	0	0
Fair value of plan assets at end of year	6	5
U.S. Stocks | Fair Value, Inputs, Level 3
Change in plan assets
Fair value of plan assets at beginning of year	1	2
Acquisitions (dispositions)	0	0
Actual return on plan assets	1	(1)
Fair value of plan assets at end of year	2	1
Private Debt | Fair Value, Inputs, Level 3
Change in plan assets
Fair value of plan assets at beginning of year	16	15
Acquisitions (dispositions)	(17)	1
Actual return on plan assets	1	0
Fair value of plan assets at end of year	$ 0	$ 16