Employee Benefits - Change in Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Change in benefit obligation
Benefit obligation at beginning of year	$ 14,800	$ 16,500
Benefit obligation at end of year	15,300	14,800	$ 16,500
Pension Plan
Change in benefit obligation
Benefit obligation at beginning of year	11,594	13,064	13,244
Service cost	56	66	63
Interest cost	436	392	409
Plan amendments	(9)	0	0
Special termination benefits charge	6	15	0
Actuarial (gain) loss	1,249	(765)	586
Benefits paid from plan assets	(1,115)	(1,178)	(1,238)
Benefit obligation at end of year	12,217	11,594	13,064
Other Postretirement Benefits Plan
Change in benefit obligation
Benefit obligation at beginning of year	2,977	3,375	3,413
Service cost	15	18	18
Interest cost	110	97	100
Participant contributions	52	54	54
Direct subsidy receipts	7	8	7
Plan amendments	0	(36)	0
Actuarial (gain) loss	180	(224)	112
Benefits paid by company	(300)	(311)	(298)
Benefits paid from plan assets	(4)	(4)	(31)
Benefit obligation at end of year	$ 3,037	$ 2,977	$ 3,375