Accumulated Other Comprehensive Income/(Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	$ (592)	$ (995)
Other comprehensive income/(loss) before reclassifications	39	68
Amounts reclassified from accumulated other comprehensive (income)/loss	20	14
Other comprehensive income/(loss)	59	82
Balance at end of period	(533)	(913)
Foreign currency translation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(587)	(994)
Other comprehensive income/(loss) before reclassifications	65	80
Amounts reclassified from accumulated other comprehensive (income)/loss	0	0
Other comprehensive income/(loss)	65	80
Balance at end of period	(522)	(914)
Derivative financial instruments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(5)	(1)
Other comprehensive income/(loss) before reclassifications	(26)	(12)
Amounts reclassified from accumulated other comprehensive (income)/loss	20	14
Other comprehensive income/(loss)	(6)	2
Balance at end of period	$ (11)	$ 1