Employee Benefits - Change in Plan Assets (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Change in plan assets
Return on plan assets	$ 1,600,000,000	$ (350,000,000)
Pension Plan
Change in plan assets
Fair value of plan assets at beginning of year	10,033,000,000	11,060,000,000	$ 10,892,000,000
Return on plan assets	1,575,000,000	(349,000,000)	1,306,000,000
Employer contributions	0	500,000,000	100,000,000
Benefits paid from plan assets	(1,115,000,000)	(1,178,000,000)	(1,238,000,000)
Fair value of plan assets at end of year	10,493,000,000	10,033,000,000	11,060,000,000
Other Postretirement Benefits Plan
Change in plan assets
Fair value of plan assets at beginning of year	18,000,000	23,000,000	53,000,000
Return on plan assets	(1,000,000)	(1,000,000)	1,000,000
Employer contributions	0	0
Benefits paid from plan assets	(4,000,000)	(4,000,000)	(31,000,000)
Fair value of plan assets at end of year	$ 13,000,000	$ 18,000,000	$ 23,000,000