Employee Benefits - Change in Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in plan assets
Return on plan assets	$ 1,200	$ 1,600
Combined Pension Plan
Change in plan assets
Fair value of plan assets at beginning of year	10,493	10,033	$ 11,060
Return on plan assets	1,210	1,575	(349)
Employer contributions	0	0	500
Benefits paid from plan assets	(1,157)	(1,115)	(1,178)
Fair value of plan assets at end of year	$ 10,546	$ 10,493	$ 10,033