Investments in Marketable Securities - Summary of Contractual Maturities of Available-for-Sale Securities (Details) $ in Millions	Sep. 30, 2015 USD ($)
Marketable Securities [Abstract]
Due within 1 Year, Amount	$ 3,367
Due after 1 Year through 5 Years, Amount	594
Due after 5 Years through 10 Years, Amount	76
Due after 10 Years, Amount	87
Mortgage-backed and asset-backed securities, fair value	133	[1]
Total, fair value	4,257
Due within 1 Year, Amortized Cost	3,367
Due after 1 Year through 5 Years, Amortized Cost	591
Due after 5 Years through 10 Years, Amortized Cost	76
Due after 10 Years, Amortized Cost	85
Mortgage-backed and asset-backed securities, amortized cost	131	[1]
Available-for-sale securities, Debt instruments, Amortized cost	$ 4,250

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