Summary of Significant Accounting Policies - Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Cash and cash equivalents	$ 3,708	$ 2,443
Restricted cash and cash equivalents	447	259
Restricted cash included within long-term investments and other assets	3	3
Total cash, cash equivalents and restricted cash and cash equivalents in the consolidated statement of cash flow	$ 4,158	$ 2,705	$ 3,646	$ 2,917