Investments in Marketable Securities (Naratives) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Mar. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	$ 2,079
Due after 1 Year through 5 Years, Amount	117
Due after 5 Years through 10 Years, Amount	119
Due after 10 Years, Amount	149
Total, fair value	4,836	4,659
Due in 1 Year or Less, Yield	0.41%
Due after 1 Year through 5 Years, Yield	2.88%
Due after 5 Years through 10 Years, Yield	3.26%
Due after 10 Years, Yield	4.33%
Total, Yield	2.52%
Due in 1 Year or Less, Amortized Cost	2,079
Due after 1 Year through 5 Years, Amortized Cost	113
Due after 5 Years through 10 Years, Amortized Cost	113
Due after 10 Years, Amortized Cost	138
Amortized cost	4,563	4,403
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
Total, Yield	4.40%
Debt instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	2,079
Due after 1 Year through 5 Years, Amount	117
Due after 5 Years through 10 Years, Amount	119
Due after 10 Years, Amount	149
Total, fair value	2,464
Due in 1 Year or Less, Yield	0.41%
Due after 1 Year through 5 Years, Yield	2.88%
Due after 5 Years through 10 Years, Yield	3.26%
Due after 10 Years, Yield	4.33%
Total, Yield	0.90%
Debt instruments [Member] | U.S. government and agency obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	9
Due after 1 Year through 5 Years, Amount	54
Due after 5 Years through 10 Years, Amount	70
Due after 10 Years, Amount	2
Total, fair value	135	108
Due in 1 Year or Less, Yield	0.07%
Due after 1 Year through 5 Years, Yield	1.33%
Due after 5 Years through 10 Years, Yield	2.08%
Due after 10 Years, Yield	3.13%
Total, Yield	1.74%
Amortized cost	131	108
Debt instruments [Member] | Municipal debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	0
Due after 1 Year through 5 Years, Amount	0
Due after 5 Years through 10 Years, Amount	1
Due after 10 Years, Amount	18
Total, fair value	19	20
Due in 1 Year or Less, Yield	0.00%
Due after 1 Year through 5 Years, Yield	0.00%
Due after 5 Years through 10 Years, Yield	5.53%
Due after 10 Years, Yield	6.00%
Total, Yield	5.97%
Amortized cost	16	17
Debt instruments [Member] | Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	1,724
Due after 1 Year through 5 Years, Amount	0
Due after 5 Years through 10 Years, Amount	0
Due after 10 Years, Amount	0
Total, fair value	1,724	1,341
Due in 1 Year or Less, Yield	0.46%
Due after 1 Year through 5 Years, Yield	0.00%
Due after 5 Years through 10 Years, Yield	0.00%
Due after 10 Years, Yield	0.00%
Total, Yield	0.46%
Amortized cost	1,724	1,341
Debt instruments [Member] | Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	344
Due after 1 Year through 5 Years, Amount	0
Due after 5 Years through 10 Years, Amount	0
Due after 10 Years, Amount	0
Total, fair value	344	633
Due in 1 Year or Less, Yield	0.13%
Due after 1 Year through 5 Years, Yield	0.00%
Due after 5 Years through 10 Years, Yield	0.00%
Due after 10 Years, Yield	0.00%
Total, Yield	0.13%
Amortized cost	344	633
Debt instruments [Member] | Foreign government debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	0
Due after 1 Year through 5 Years, Amount	3
Due after 5 Years through 10 Years, Amount	0
Due after 10 Years, Amount	0
Total, fair value	3	3
Due in 1 Year or Less, Yield	0.00%
Due after 1 Year through 5 Years, Yield	2.93%
Due after 5 Years through 10 Years, Yield	0.00%
Due after 10 Years, Yield	0.00%
Total, Yield	2.93%
Amortized cost		3
Debt instruments [Member] | Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	2
Due after 1 Year through 5 Years, Amount	53
Due after 5 Years through 10 Years, Amount	39
Due after 10 Years, Amount	1
Total, fair value	95	107
Due in 1 Year or Less, Yield	3.70%
Due after 1 Year through 5 Years, Yield	4.38%
Due after 5 Years through 10 Years, Yield	5.19%
Due after 10 Years, Yield	7.86%
Total, Yield	4.70%
Amortized cost	89	100
Debt instruments [Member] | U.S. government agency mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	0
Due after 1 Year through 5 Years, Amount	0
Due after 5 Years through 10 Years, Amount	6
Due after 10 Years, Amount	94
Total, fair value	100	105
Due in 1 Year or Less, Yield	0.00%
Due after 1 Year through 5 Years, Yield	0.00%
Due after 5 Years through 10 Years, Yield	4.23%
Due after 10 Years, Yield	3.95%
Total, Yield	3.97%
Amortized cost	94	100
Debt instruments [Member] | Non-agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	0
Due after 1 Year through 5 Years, Amount	0
Due after 5 Years through 10 Years, Amount	0
Due after 10 Years, Amount	7
Total, fair value	7	8
Due in 1 Year or Less, Yield	0.00%
Due after 1 Year through 5 Years, Yield	0.00%
Due after 5 Years through 10 Years, Yield	0.00%
Due after 10 Years, Yield	8.20%
Total, Yield	8.20%
Amortized cost	6	7
Debt instruments [Member] | Non-agency commercial mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	0
Due after 1 Year through 5 Years, Amount	3
Due after 5 Years through 10 Years, Amount	1
Due after 10 Years, Amount	21
Total, fair value	25	26
Due in 1 Year or Less, Yield	0.00%
Due after 1 Year through 5 Years, Yield	3.59%
Due after 5 Years through 10 Years, Yield	1.04%
Due after 10 Years, Yield	4.24%
Total, Yield	4.05%
Amortized cost	24	25
Debt instruments [Member] | Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	0
Due after 1 Year through 5 Years, Amount	4
Due after 5 Years through 10 Years, Amount	2
Due after 10 Years, Amount	6
Total, fair value	12	13
Due in 1 Year or Less, Yield	0.00%
Due after 1 Year through 5 Years, Yield	2.45%
Due after 5 Years through 10 Years, Yield	1.13%
Due after 10 Years, Yield	1.38%
Total, Yield	1.68%
Amortized cost	12	13
Equity instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	2,372
Total, Yield	4.20%
Equity instruments [Member] | Fixed income mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	1,933
Equity instruments [Member] | Equity mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	439	451
Total, Yield	3.30%
Amortized cost	$ 271	$ 268