Investments in Marketable Securities - Summary of Contractual Maturities of Available-for-Sale Securities (Details) $ in Millions	Dec. 31, 2016 USD ($)
Marketable Securities [Abstract]
Due within 1 Year, Amount	$ 2,563
Due after 1 Year through 5 Years, Amount	741
Due after 5 Years through 10 Years, Amount	131
Due after 10 Years, Amount	80
Mortgage-backed and asset-backed securities, fair value	113	[1]
Total, fair value	3,628
Due within 1 Year, Amortized Cost	2,563
Due after 1 Year through 5 Years, Amortized Cost	742
Due after 5 Years through 10 Years, Amortized Cost	132
Due after 10 Years, Amortized Cost	82
Mortgage-backed and asset-backed securities, amortized cost	113	[1]
Available-for-sale securities, Debt instruments, Amortized cost	$ 3,632

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