Employee Benefit Plans - Change in Benefit Obligation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation — beginning of year	$ 4,000	$ 4,376
Service cost	4	4	$ 4
Interest cost	159	142	170
Plan participants’ contributions	0	0
Actuarial loss (gain)	48	(6)
Amendments	0	0
Changes in assumptions	488	(329)
Benefits and expenses paid	(201)	(186)
Retiree drug subsidy	0	0
Foreign exchange adjustment	0	(1)
Benefit obligation — end of year	4,498	4,000	4,376
Other Postretirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation — beginning of year	220	256
Service cost	0	0
Interest cost	8	7	8
Plan participants’ contributions	13	11
Actuarial loss (gain)	6	0
Amendments	(2)	0
Changes in assumptions	19	(11)
Benefits and expenses paid	(41)	(45)
Retiree drug subsidy	0	2
Foreign exchange adjustment	0	0
Benefit obligation — end of year	$ 223	$ 220	$ 256