Investments in Marketable Securities - Summary of Contractual Maturities of Available-for-Sale Securities (Details) $ in Millions	Dec. 31, 2015 USD ($)
Marketable Securities [Abstract]
Due within 1 Year, Amount	$ 4,544
Due after 1 Year through 5 Years, Amount	1,278
Due after 5 Years through 10 Years, Amount	80
Due after 10 Years, Amount	95
Mortgage-backed and asset-backed securities, fair value	131	[1]
Total, fair value	6,128
Due within 1 Year, Amortized Cost	4,550
Due after 1 Year through 5 Years, Amortized Cost	1,281
Due after 5 Years through 10 Years, Amortized Cost	81
Due after 10 Years, Amortized Cost	93
Mortgage-backed and asset-backed securities, amortized cost	130	[1]
Available-for-sale securities, Debt instruments, Amortized cost	$ 6,135

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