Investments in Marketable Securities - Summary of Contractual Maturities of Available-for-Sale Securities (Details) $ in Millions	Dec. 31, 2017 USD ($)
Available-for-sale debt instruments, Amortized Cost
Due within 1 Year, Amortized Cost	$ 1,948
Due after 1 Year through 5 Years, Amortized Cost	1,617
Due after 5 Years through 10 Years, Amortized Cost	144
Due after 10 Years, Amortized Cost	91
Mortgage-backed and asset-backed securities, amortized cost	113	[1]
Available-for-sale securities, Debt instruments, Amortized cost	3,913
Available-for-sale debt instruments, Fair Value
Due within 1 Year, Amount	1,944
Due after 1 Year through 5 Years, Amount	1,607
Due after 5 Years through 10 Years, Amount	144
Due after 10 Years, Amount	93
Mortgage-backed and asset-backed securities, fair value	114	[1]
Total, fair value	$ 3,902

[1] Mortgage-backed and asset-backed securities are shown separately from other maturity groupings as these securities have multiple maturity dates.