Goodwill and Other Intangible Assets - Finite-Lived Intangible Assets and Liabilities (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2018	Dec. 31, 2017
Finite-lived Intangible Assets [Roll Forward]
Balance at December 31, 2017		$ 3,882
Intangible assets acquired		3
Intangible assets disposed		(7)
Amortization (i)		(409)
Foreign exchange		(40)
Balance at September 30, 2018		3,429
Finite-lived intangible assets, gross carrying amount		5,324	$ 5,408
Finite-lived intangible assets, accumulated amortization		(1,895)	(1,526)
Finite-lived intangible liabilities, gross carrying amount		34	34
Finite-lived intangible liabilities, accumulated amortization		(12)	(8)
Client relationships [Member]
Finite-lived Intangible Assets [Roll Forward]
Balance at December 31, 2017		2,342
Intangible assets acquired		3
Intangible assets disposed		(7)
Amortization (i)		(263)
Foreign exchange		(28)
Balance at September 30, 2018		2,047
Finite-lived intangible assets, gross carrying amount		3,397	3,462
Finite-lived intangible assets, accumulated amortization		(1,350)	(1,120)
Management contracts [Member]
Finite-lived Intangible Assets [Roll Forward]
Balance at December 31, 2017		56
Intangible assets acquired		0
Intangible assets disposed		0
Amortization (i)		(3)
Foreign exchange		(4)
Balance at September 30, 2018		49
Finite-lived intangible assets, gross carrying amount		63	68
Finite-lived intangible assets, accumulated amortization		(14)	(12)
Software [Member]
Finite-lived Intangible Assets [Roll Forward]
Balance at December 31, 2017		473
Intangible assets acquired		0
Intangible assets disposed		0
Amortization (i)		(106)
Foreign exchange		(4)
Balance at September 30, 2018		363
Finite-lived intangible assets, gross carrying amount		754	764
Finite-lived intangible assets, accumulated amortization		(391)	(291)
Trademark and trade name [Member]
Finite-lived Intangible Assets [Roll Forward]
Balance at December 31, 2017		966
Intangible assets acquired		0
Intangible assets disposed		0
Amortization (i)		(33)
Foreign exchange		(3)
Balance at September 30, 2018		930
Finite-lived intangible assets, gross carrying amount		1,053	1,055
Finite-lived intangible assets, accumulated amortization		(123)	(89)
Product [Member]
Finite-lived Intangible Assets [Roll Forward]
Balance at December 31, 2017		33
Intangible assets acquired		0
Intangible assets disposed		0
Amortization (i)		(2)
Foreign exchange		(2)
Balance at September 30, 2018		29
Finite-lived intangible assets, gross carrying amount		37	39
Finite-lived intangible assets, accumulated amortization		(8)	(6)
Favorable agreements [Member]
Finite-lived Intangible Assets [Roll Forward]
Balance at December 31, 2017		10
Intangible assets acquired		0
Intangible assets disposed		0
Amortization (i)	[1]	(1)
Foreign exchange		1
Balance at September 30, 2018		10
Finite-lived intangible assets, gross carrying amount		15	14
Finite-lived intangible assets, accumulated amortization		(5)	(4)
Other [Member]
Finite-lived Intangible Assets [Roll Forward]
Balance at December 31, 2017		2
Intangible assets acquired		0
Intangible assets disposed		0
Amortization (i)		(1)
Foreign exchange		0
Balance at September 30, 2018		1
Finite-lived intangible assets, gross carrying amount		5	6
Finite-lived intangible assets, accumulated amortization		$ (4)	$ (4)

[1] Amortization associated with favorable lease agreements is recorded in Other operating expenses in the condensed consolidated statements of comprehensive income.