Investments in Marketable Securities - Summary of Contractual Maturities of Available-for-Sale Securities (Details) $ in Millions	Mar. 31, 2018 USD ($)
Available-for-sale debt instruments, Amortized Cost
Due within 1 Year, Amortized Cost	$ 2,089
Due after 1 Year through 5 Years, Amortized Cost	1,247
Due after 5 Years through 10 Years, Amortized Cost	151
Due after 10 Years, Amortized Cost	74
Mortgage-backed and asset-backed securities, amortized cost	108	[1]
Available-for-sale securities, Debt instruments, Amortized cost	3,669
Available-for-sale debt instruments, Fair Value
Due within 1 Year, Amount	2,082
Due after 1 Year through 5 Years, Amount	1,235
Due after 5 Years through 10 Years, Amount	146
Due after 10 Years, Amount	73
Mortgage-backed and asset-backed securities, fair value	109	[1]
Total, fair value	$ 3,645

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