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Basis of Presentation - Schedule of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 1,949		$ 2,745
Increase (decrease) of cash and cash equivalents	(796)
Cash, cash equivalents and restricted cash	3,120	$ 2,728	3,916	$ 2,888
Decrease in cash, cash equivalents and restricted cash	(796)	$ (160)
Prepaid expenses and other
Cash and Cash Equivalents [Line Items]
Restricted cash	1,055		1,047
Increase (decrease) in restricted cash	8
Other assets
Cash and Cash Equivalents [Line Items]
Restricted cash	116		$ 124
Increase (decrease) in restricted cash	$ (8)