Summary of Significant Accounting Policies - Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Cash and cash equivalents	$ 3,905	$ 3,315
Restricted cash and cash equivalents	813	779
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	$ 4,721	$ 4,097	$ 4,158	$ 2,705