Investments in Marketable Securities (Naratives) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Mar. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	$ 2,360
Due after 1 Year through 5 Years, Amount	112
Due after 5 Years through 10 Years, Amount	128
Due after 10 Years, Amount	159
Total, fair value	5,160	4,659
Due in 1 Year or Less, Yield	0.57%
Due after 1 Year through 5 Years, Yield	2.97%
Due after 5 Years through 10 Years, Yield	2.85%
Due after 10 Years, Yield	3.95%
Total, Yield	2.51%
Due in 1 Year or Less, Amortized Cost	2,359
Due after 1 Year through 5 Years, Amortized Cost	107
Due after 5 Years through 10 Years, Amortized Cost	122
Due after 10 Years, Amortized Cost	147
Amortized cost	4,824	4,403
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
Total, Yield	4.47%
Debt instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	2,360
Due after 1 Year through 5 Years, Amount	112
Due after 5 Years through 10 Years, Amount	128
Due after 10 Years, Amount	159
Total, fair value	2,759
Due in 1 Year or Less, Yield	0.57%
Due after 1 Year through 5 Years, Yield	2.97%
Due after 5 Years through 10 Years, Yield	2.85%
Due after 10 Years, Yield	3.95%
Total, Yield	0.96%
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	2
Due after 1 Year through 5 Years, Amount	49
Due after 5 Years through 10 Years, Amount	80
Due after 10 Years, Amount	11
Total, fair value	142	108
Due in 1 Year or Less, Yield	0.05%
Due after 1 Year through 5 Years, Yield	1.29%
Due after 5 Years through 10 Years, Yield	1.75%
Due after 10 Years, Yield	1.84%
Total, Yield	1.52%
Amortized cost	138	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	3
Due after 5 Years through 10 Years, Amount	1
Due after 10 Years, Amount	18
Total, fair value	22	20
Due in 1 Year or Less, Yield	0.00%
Due after 1 Year through 5 Years, Yield	4.38%
Due after 5 Years through 10 Years, Yield	5.54%
Due after 10 Years, Yield	6.00%
Total, Yield	5.76%
Amortized cost	18	17
Debt instruments [Member] | Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	1,845
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	1,845	1,341
Due in 1 Year or Less, Yield	0.67%
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	0.67%
Amortized cost	1,844	1,341
Debt instruments [Member] | Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	511
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	511	633
Due in 1 Year or Less, Yield	0.18%
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	0.18%
Amortized cost	511	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	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	52
Due after 5 Years through 10 Years, Amount	37
Due after 10 Years, Amount	1
Total, fair value	92	107
Due in 1 Year or Less, Yield	4.85%
Due after 1 Year through 5 Years, Yield	4.41%
Due after 5 Years through 10 Years, Yield	5.13%
Due after 10 Years, Yield	7.86%
Total, Yield	4.72%
Amortized cost	85	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	5
Due after 10 Years, Amount	99
Total, fair value	104	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	4.29%
Due after 10 Years, Yield	3.63%
Total, Yield	3.67%
Amortized cost	98	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	8.07%
Total, Yield	8.07%
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	4
Due after 5 Years through 10 Years, Amount	2
Due after 10 Years, Amount	18
Total, fair value	24	26
Due in 1 Year or Less, Yield	0.00%
Due after 1 Year through 5 Years, Yield	3.45%
Due after 5 Years through 10 Years, Yield	0.86%
Due after 10 Years, Yield	3.91%
Total, Yield	3.62%
Amortized cost	23	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	1
Due after 5 Years through 10 Years, Amount	3
Due after 10 Years, Amount	5
Total, fair value	9	13
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	1.01%
Due after 10 Years, Yield	1.56%
Total, Yield	1.67%
Amortized cost	9	13
Equity instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	2,401
Total, Yield	4.29%
Equity instruments [Member] | Fixed income mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	1,935
Equity instruments [Member] | Equity mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	466	451
Total, Yield	3.53%
Amortized cost	$ 273	$ 268