Investments in Marketable Securities - Summary of Contractual Maturities of Available-for-Sale Securities (Details) $ in Millions	Jun. 30, 2017 USD ($)
Available-for-sale debt instruments, Amortized Cost
Due within 1 Year, Amortized Cost	$ 3,351
Due after 1 Year through 5 Years, Amortized Cost	814
Due after 5 Years through 10 Years, Amortized Cost	135
Due after 10 Years, Amortized Cost	87
Mortgage-backed and asset-backed securities, amortized cost	121	[1]
Available-for-sale securities, Debt instruments, Amortized cost	4,508
Available-for-sale debt instruments, Fair Value
Due within 1 Year, Amount	3,349
Due after 1 Year through 5 Years, Amount	813
Due after 5 Years through 10 Years, Amount	135
Due after 10 Years, Amount	88
Mortgage-backed and asset-backed securities, fair value	122	[1]
Total, fair value	$ 4,507

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