Employee Benefits - Change in Plan Assets Measured at Fair Value (Details) - Pension Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Change in plan assets
Fair value of plan assets at beginning of year	$ 10,033	$ 11,060
Fair value of plan assets at end of year	10,493	10,033
Fair Value, Inputs, Level 3
Change in plan assets
Fair value of plan assets at beginning of year	24	27
Acquisitions (dispositions)	(1)	(3)
Actual return on plan assets	(1)	0
Fair value of plan assets at end of year	22	24
High Yield Bonds | Fair Value, Inputs, Level 3
Change in plan assets
Fair value of plan assets at beginning of year	7	7
Acquisitions (dispositions)	(2)	0
Actual return on plan assets	0	0
Fair value of plan assets at end of year	5	7
Emerging Market Bonds | Fair Value, Inputs, Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0	1
Acquisitions (dispositions)	0	0
Actual return on plan assets	0	(1)
Fair value of plan assets at end of year	0	0
Domestic Equity Securities | Fair Value, Inputs, Level 3
Change in plan assets
Fair value of plan assets at beginning of year	2	3
Acquisitions (dispositions)	0	(2)
Actual return on plan assets	(1)	1
Fair value of plan assets at end of year	1	2
Private Debt | Fair Value, Inputs, Level 3
Change in plan assets
Fair value of plan assets at beginning of year	15	15
Acquisitions (dispositions)	1	0
Actual return on plan assets	0	0
Fair value of plan assets at end of year	16	15
Cash and Cash Equivalents | Fair Value, Inputs, Level 3
Change in plan assets
Fair value of plan assets at beginning of year	0	1
Acquisitions (dispositions)	0	(1)
Actual return on plan assets	0	0
Fair value of plan assets at end of year	$ 0	$ 0