Investments in Marketable Securities (Naratives) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Mar. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	$ 2,427
Due after 1 Year through 5 Years, Amount	172
Due after 5 Years through 10 Years, Amount	73
Mortgage-backed and asset-backed securities, fair value	137	[1]
Due after 10 Years, Amount	54
Total, fair value	5,071		5,389
Due in 1 Year or Less, Amortized Cost	2,427
Due after 1 Year through 5 Years, Amortized Cost	171
Due after 5 Years through 10 Years, Amortized Cost	72
Due after 10 Years, Amortized Cost	51
Mortgage-backed and asset-backed securities, amortized cost	134	[1]
Total, amortized cost	4,733		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	1,700		1,800
Debt instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	2,863
Total, amortized cost	2,855
Debt instruments [Member] | U.S. government and agency obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	2,246		652
Total, amortized cost	2,246		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	30		1,599
Total, amortized cost	30		1,599
Debt instruments [Member] | Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	271		507
Total, amortized cost	271		507
Debt instruments [Member] | Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	167		169
Total, amortized cost	163		164
Debt instruments [Member] | U.S. government agency mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	57		60
Total, amortized cost	55		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	46		43
Total, amortized cost	46		44
Debt instruments [Member] | Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	30		27
Total, amortized cost	30		27
Equity instruments [Member] | Equity mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	473		481
Total, amortized cost	$ 203		$ 217

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