Employee Benefits - Change in Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Change in benefit obligation
Benefit obligation at beginning of year	$ 16,500	$ 16,700
Benefit obligation at end of year	14,800	16,500	$ 16,700
Pension Plan
Change in benefit obligation
Benefit obligation at beginning of year	13,064	13,244	13,287
Service cost	66	63	64
Interest cost	392	409	425
Plan amendments	0	0	2
Special termination benefits charge	15	0	13
Actuarial (gain) loss	(765)	586	487
Benefits paid by company	0	0	0
Benefits paid from plan assets	(1,178)	(1,238)	(1,034)
Benefit obligation at end of year	11,594	13,064	13,244
Other Postretirement Benefits Plan
Change in benefit obligation
Benefit obligation at beginning of year	3,375	3,413	3,567
Service cost	18	18	19
Interest cost	97	100	111
Participant contributions	54	54	57
Direct subsidy receipts	8	7	5
Plan amendments	(36)	0	0
Special termination benefits charge	0	0	3
Actuarial (gain) loss	(224)	112	(13)
Benefits paid by company	(311)	(298)	(191)
Benefits paid from plan assets	(4)	(31)	(145)
Benefit obligation at end of year	$ 2,977	$ 3,375	$ 3,413