Investments in Marketable Securities - Summary of Contractual Maturities of Available-for-Sale Securities (Details) $ in Millions	Mar. 31, 2017 USD ($)
Available-for-sale debt instruments, Amortized Cost
Due within 1 Year, Amortized Cost	$ 2,303
Due after 1 Year through 5 Years, Amortized Cost	926
Due after 5 Years through 10 Years, Amortized Cost	126
Due after 10 Years, Amortized Cost	92
Mortgage-backed and asset-backed securities, amortized cost	115	[1]
Available-for-sale securities, Debt instruments, Amortized cost	3,562
Available-for-sale debt instruments, Fair Value
Due within 1 Year, Amount	2,302
Due after 1 Year through 5 Years, Amount	926
Due after 5 Years through 10 Years, Amount	125
Due after 10 Years, Amount	90
Mortgage-backed and asset-backed securities, fair value	115	[1]
Total, fair value	$ 3,558

[1] Mortgage-backed and asset-backed securities are shown separately from other maturity groupings as these securities do not have a single maturity date.