Employee Benefits - Change in Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Combined Pension Plan
Change in benefit obligation
Benefit obligation at beginning of year	$ 12,217	$ 11,594	$ 13,064
Service cost	59	56	66
Interest cost	324	436	392
Plan amendments	(3)	(9)	0
Special termination benefits charge	13	6	15
Actuarial loss (gain)	749	1,249	(765)
Benefits paid from plan assets	(1,157)	(1,115)	(1,178)
Benefit obligation at end of year	12,202	12,217	11,594
Post-Retirement Benefit Plans
Change in benefit obligation
Benefit obligation at beginning of year	3,037	2,977	3,375
Service cost	14	15	18
Interest cost	69	110	97
Participant contributions	46	52	54
Direct subsidy receipts	6	7	8
Plan amendments	0	0	(36)
Actuarial loss (gain)	134	180	(224)
Curtailment loss	4	0	0
Benefits paid by company	(255)	(300)	(311)
Benefits paid from plan assets	(7)	(4)	(4)
Benefit obligation at end of year	$ 3,048	$ 3,037	$ 2,977