Benefit Plans - Changes in Projected Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Service cost	$ 238	$ 200	$ 186
Interest cost	201	192	181
Qualified Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation, beginning balance	4,939	3,939
Service cost	222	188
Interest cost	186	173
Actuarial (gain) loss	(537)	576
Benefits paid	(113)	(102)
Plan amendments	0	165
Projected benefit obligation, ending balance	4,697	4,939	3,939
Accumulated benefit obligation, end of year	$ 4,035	$ 4,198
Weighted average assumptions used to determine projected benefit obligations:
Weighted average assumed discount rate	4.43%	3.79%
Assumed rate of annual compensation increases	4.50%	4.50%
Nonqualified Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation, beginning balance	$ 387	$ 426
Service cost	16	12
Interest cost	15	19
Actuarial (gain) loss	(19)	77
Benefits paid	(13)	(12)
Plan amendments	0	(135)
Projected benefit obligation, ending balance	386	387	$ 426
Accumulated benefit obligation, end of year	$ 293	$ 288
Weighted average assumptions used to determine projected benefit obligations:
Weighted average assumed discount rate	4.43%	3.79%
Assumed rate of annual compensation increases	4.50%	4.50%