Employee Benefit Plans - Level 3 Rollforward (Details) - UNITED KINGDOM - Pension Plan [Member] - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Beginning balance		$ 2,368		$ 2,169
Foreign Currency Exchange Rate Changes		(120)		(207)
Ending balance		1,989		2,368		$ 2,169
Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Beginning balance	[1]	230
Ending balance	[1]	239		230
Real Estate Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Beginning balance		230
Ending balance		239		230
Real Estate Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Beginning balance		230	[1]	105		204
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Still Held		23		6		10
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Purchase, Sale, and Settlement		0		104		(80)
Foreign Currency Exchange Rate Changes		(14)		15		(29)
Ending balance		$ 239	[1]	$ 230	[1]	$ 105

[1] Refer to Note 14 for additional discussion regarding the three levels of the fair value hierarchy.