Employee Benefit Plans - Level 3 Rollforward (Details) - UNITED KINGDOM - Pension Plan [Member] - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Beginning balance		$ 2,169		$ 2,276
Foreign Currency Exchange Rate Changes		207		397
Ending balance		2,368		2,169		$ 2,276
Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Beginning balance	[1]	105
Ending balance	[1]	230		105
Real Estate Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Beginning balance		105
Ending balance		230		105
Real Estate Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Beginning balance		105	[1]	204		199
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Still Held		6		10		18
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Purchases, Sales, and Settlements		104		(80)		0
Foreign Currency Exchange Rate Changes		15		(29)		(13)
Ending balance		$ 230	[1]	$ 105	[1]	$ 204

[1] Refer to Note 15 for additional discussion regarding the three levels of the fair value hierarchy.