Investments in Marketable Securities (Naratives) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014		Mar. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	$ 2,251
Due after 1 Year through 5 Years, Amount	156
Due after 5 Years through 10 Years, Amount	67
Mortgage-backed and asset-backed securities, fair value	133	[1]
Due after 10 Years, Amount	57
Total, fair value	5,036		5,389
Due in 1 Year or Less, Amortized Cost	2,249
Due after 1 Year through 5 Years, Amortized Cost	154
Due after 5 Years through 10 Years, Amortized Cost	66
Due after 10 Years, Amortized Cost	53
Mortgage-backed and asset-backed securities, amortized cost	130	[1]
Total, amortized cost	4,655		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
Fixed income mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	1,900		1,800
Debt instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	2,664
Total, amortized cost	2,652
Debt instruments [Member] | U.S. government and agency obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	1,132		652
Total, amortized cost	1,130		652
Debt instruments [Member] | Municipal debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	12		11
Total, amortized cost	11		10
Debt instruments [Member] | Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	914		1,599
Total, amortized cost	914		1,599
Debt instruments [Member] | Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	329		507
Total, amortized cost	329		507
Debt instruments [Member] | Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	144		169
Total, amortized cost	138		164
Debt instruments [Member] | U.S. government agency mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	59		60
Total, amortized cost	57		60
Debt instruments [Member] | Non-agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	5		5
Total, amortized cost	4		4
Debt instruments [Member] | Non-agency commercial mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	40		43
Total, amortized cost	40		44
Debt instruments [Member] | Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	29		27
Total, amortized cost	29		27
Equity instruments [Member] | Equity mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	487		481
Total, amortized cost	$ 210		$ 217

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