Benefit Plans - Changes in Projected Benefit Obligation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Service cost	$ 200	$ 186	$ 176
Interest cost	192	181	157
Qualified Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation, beginning of year	3,939	3,473
Service cost	188	174
Interest cost	173	163
Actuarial (gain) loss	576	152
Benefits paid	(102)	(94)
Plan amendments	165	0
Acquisitions	0	71
Projected benefit obligation, end of year	4,939	3,939	3,473
Accumulated benefit obligation, end of year	4,198	3,403
Nonqualified Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation, beginning of year	426	392
Service cost	12	12
Interest cost	19	18
Actuarial (gain) loss	77	15
Benefits paid	(12)	(11)
Plan amendments	(135)	0
Acquisitions	0	0
Projected benefit obligation, end of year	387	426	$ 392
Accumulated benefit obligation, end of year	$ 288	$ 363