Investments in Marketable Securities - Summary of Contractual Maturities of Available-for-Sale Securities (Details) $ in Millions	Sep. 30, 2017 USD ($)
Available-for-sale debt instruments, Amortized Cost
Due within 1 Year, Amortized Cost	$ 1,980
Due after 1 Year through 5 Years, Amortized Cost	1,606
Due after 5 Years through 10 Years, Amortized Cost	142
Due after 10 Years, Amortized Cost	84
Mortgage-backed and asset-backed securities, amortized cost	116	[1]
Available-for-sale securities, Debt instruments, Amortized cost	3,928
Available-for-sale debt instruments, Fair Value
Due within 1 Year, Amount	1,978
Due after 1 Year through 5 Years, Amount	1,604
Due after 5 Years through 10 Years, Amount	142
Due after 10 Years, Amount	86
Mortgage-backed and asset-backed securities, fair value	118	[1]
Total, fair value	$ 3,928

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