Investments - Short-Term Investments and Cash Equivalents (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Corporate securities
Short Term Investments And Cash Equivalents [Line Items]
Short-term investments and cash equivalents	$ 1,954	$ 1,985
Federal government securities
Short Term Investments And Cash Equivalents [Line Items]
Short-term investments and cash equivalents	110	472
Foreign government securities
Short Term Investments And Cash Equivalents [Line Items]
Short-term investments and cash equivalents	25	65
Money market funds
Short Term Investments And Cash Equivalents [Line Items]
Short-term investments and cash equivalents	$ 963	$ 631