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Basis of Presentation - Schedule of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 1,750		$ 2,079
Increase (decrease) of cash and cash equivalents	(329)
Cash, cash equivalents and restricted cash	2,728	$ 5,124	2,888	$ 3,177
Increase (decrease) in cash, cash equivalents and restricted cash	(160)	$ 1,947
Prepaid expenses and other
Cash and Cash Equivalents [Line Items]
Restricted cash	851		693
Increase (decrease) in restricted cash	158
Other assets
Cash and Cash Equivalents [Line Items]
Restricted cash	127		$ 116
Increase (decrease) in restricted cash	$ 11