Investments - Equity Method Investments (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Equity investments without readily determinable fair values	$ 859		$ 872
Equity investments under the equity method of accounting	9		13
Total equity investments	868		$ 885
Cash investment	$ 10,365	$ 7,603