Significant Accounting Policies - Cash and Cash Equivalents (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Cash and cash equivalents	$ 2,365	$ 2,452	$ 3,224
Restricted cash and cash equivalents	58		74
Total cash, cash equivalents, and restricted cash and cash equivalents presented in the consolidated statements of cash flows	$ 2,423	$ 2,508	$ 3,298	$ 2,926