Investments in Marketable Securities - Summary of Contractual Maturities of Available-for-Sale Securities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015
Marketable Securities [Abstract]
Due in 1 Year or Less, Amount	$ 4,451
Due after 1 Year through 5 Years, Amount	124
Due after 5 Years through 10 Years, Amount	85
Due after 10 Years, Amount	68
Mortgage-backed and asset-backed securities, fair value	146	[1]
Total, fair value	4,874
Due in 1 Year or Less, Amortized Cost	4,452
Due after 1 Year through 5 Years, Amortized Cost	121
Due after 5 Years through 10 Years, Amortized Cost	82
Due after 10 Years, Amortized Cost	62
Mortgage-backed and asset-backed securities, amortized cost	142	[1]
Available-for-sale securities, Debt instruments, Amortized cost	$ 4,859

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