Employee Benefits - Change in Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Combined Pension Plan
Change in benefit obligation
Benefit obligation at beginning of year	$ 12,202	$ 12,217	$ 11,594
Service cost	56	59	56
Interest cost	201	324	436
Plan amendments	(13)	(3)	(9)
Special termination benefits charge	6	13	6
Actuarial (gain) loss	(337)	749	1,249
Benefits paid from plan assets	(766)	(1,157)	(1,115)
Settlement payments and annuity purchase	(1,671)	0	0
Benefit obligation at end of year	9,678	12,202	12,217
Post-Retirement Benefit Plans
Change in benefit obligation
Benefit obligation at beginning of year	3,048	3,037	2,977
Service cost	14	14	15
Interest cost	47	69	110
Participant contributions	41	46	52
Direct subsidy receipts	3	6	7
Actuarial (gain) loss	(125)	134	180
Curtailment loss	0	4	0
Benefits paid by company	(247)	(255)	(300)
Benefits paid from plan assets	0	(7)	(4)
Benefit obligation at end of year	$ 2,781	$ 3,048	$ 3,037