Investments in Marketable Securities - Summary of Contractual Maturities of Available-for-Sale Securities (Details) $ in Millions	Sep. 30, 2016 USD ($)
Marketable Securities [Abstract]
Due within 1 Year, Amount	$ 3,194
Due after 1 Year through 5 Years, Amount	732
Due after 5 Years through 10 Years, Amount	119
Due after 10 Years, Amount	75
Mortgage-backed and asset-backed securities, fair value	129	[1]
Total, fair value	4,249
Due within 1 Year, Amortized Cost	3,193
Due after 1 Year through 5 Years, Amortized Cost	729
Due after 5 Years through 10 Years, Amortized Cost	116
Due after 10 Years, Amortized Cost	71
Mortgage-backed and asset-backed securities, amortized cost	126	[1]
Available-for-sale securities, Debt instruments, Amortized cost	$ 4,235

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