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Basis of Presentation - Schedule of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 2,538		$ 2,079
Increase (decrease) of cash and cash equivalents	459
Cash, cash equivalents and restricted cash	3,575	$ 5,068	2,888	$ 3,177
Increase in cash, cash equivalents and restricted cash	687	$ 1,891
Prepaid expenses and other
Cash and Cash Equivalents [Line Items]
Restricted cash	915		693
Increase (decrease) in restricted cash	222
Other assets
Cash and Cash Equivalents [Line Items]
Restricted cash	122		$ 116
Increase (decrease) in restricted cash	$ 6