Investments in Marketable Securities (Naratives) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Mar. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	$ 1,927
Due after 1 Year through 5 Years, Amount	136
Due after 5 Years through 10 Years, Amount	83
Mortgage-backed and asset-backed securities, fair value	129	[1]
Due after 10 Years, Amount	60
Total, fair value	4,571		5,389
Due in 1 Year or Less, Amortized Cost	1,929
Due after 1 Year through 5 Years, Amortized Cost	135
Due after 5 Years through 10 Years, Amortized Cost	81
Due after 10 Years, Amortized Cost	54
Mortgage-backed and asset-backed securities, amortized cost	126	[1]
Total, amortized cost	4,236		5,065
Debt securities on deposit with state insurance authorities	6		6
Debt securities with amortized cost on deposit with state insurance authorities	6		6
Debt instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	2,335
Total, amortized cost	2,325
Debt instruments [Member] | U.S. government and agency obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	2,010		652
Total, amortized cost	2,010		652
Debt instruments [Member] | Municipal debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	12		11
Total, amortized cost	10		10
Debt instruments [Member] | Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value			1,599
Total, amortized cost			1,599
Debt instruments [Member] | Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value			507
Total, amortized cost			507
Debt instruments [Member] | Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	184		169
Total, amortized cost	179		164
Debt instruments [Member] | U.S. government agency mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	55		60
Total, amortized cost	53		60
Debt instruments [Member] | Non-agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	4		5
Total, amortized cost	3		4
Debt instruments [Member] | Non-agency commercial mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	45		43
Total, amortized cost	45		44
Debt instruments [Member] | Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	25		27
Total, amortized cost	25		27
Equity instruments [Member] | Equity mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	476		481
Total, amortized cost	$ 196		$ 217

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