Employee Benefit Plans - Change in Benefit Obligations and Fair Value of Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
United States
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of year	$ 1,659	$ 1,821
Interest cost	66	59	$ 77
Plan amendment	0	0
Actuarial loss (gain)	214	(147)
Benefits paid	(77)	(74)
Foreign currency exchange rate changes	0	0
Projected benefit obligation at end of year	1,862	1,659	1,821
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	1,582	1,764
Actual return on plan assets	353	(113)
Employer contributions	5	5
Benefits paid	(77)	(74)
Foreign currency exchange rate changes	0	0
Fair value of plan assets at end of year	1,863	1,582	1,764
U.K. Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of year	1,164	1,305
Interest cost	29	28	34
Plan amendment	0	19
Actuarial loss (gain)	136	(62)
Benefits paid	(56)	(56)
Foreign currency exchange rate changes	50	(70)
Projected benefit obligation at end of year	1,323	1,164	1,305
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	1,227	1,390
Actual return on plan assets	138	(35)
Employer contributions	2	3
Benefits paid	(56)	(56)
Foreign currency exchange rate changes	51	(75)
Fair value of plan assets at end of year	$ 1,362	$ 1,227	$ 1,390