Investments in Marketable Securities (Naratives) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Mar. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	$ 2,308
Due after 1 Year through 5 Years, Amount	114
Due after 5 Years through 10 Years, Amount	122
Due after 10 Years, Amount	151
Total, fair value	5,109	4,659
Due in 1 Year or Less, Yield	0.61%
Due after 1 Year through 5 Years, Yield	2.87%
Due after 5 Years through 10 Years, Yield	2.91%
Due after 10 Years, Yield	3.97%
Total, Yield	3.36%
Due in 1 Year or Less, Amortized Cost	2,307
Due after 1 Year through 5 Years, Amortized Cost	109
Due after 5 Years through 10 Years, Amortized Cost	115
Due after 10 Years, Amortized Cost	142
Amortized cost	4,808	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	2,000	1,800
Debt instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	2,308
Due after 1 Year through 5 Years, Amount	114
Due after 5 Years through 10 Years, Amount	122
Due after 10 Years, Amount	151
Total, fair value	2,695
Due in 1 Year or Less, Yield	0.61%
Due after 1 Year through 5 Years, Yield	2.87%
Due after 5 Years through 10 Years, Yield	2.91%
Due after 10 Years, Yield	3.97%
Total, Yield	1.00%
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	12
Due after 1 Year through 5 Years, Amount	55
Due after 5 Years through 10 Years, Amount	74
Due after 10 Years, Amount	11
Total, fair value	152	108
Due in 1 Year or Less, Yield	0.08%
Due after 1 Year through 5 Years, Yield	1.43%
Due after 5 Years through 10 Years, Yield	1.71%
Due after 10 Years, Yield	2.34%
Total, Yield	1.56%
Amortized cost	148	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	3.86%
Due after 5 Years through 10 Years, Yield	5.54%
Due after 10 Years, Yield	5.83%
Total, Yield	5.53%
Amortized cost	19	17
Debt instruments [Member] | Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	2,082
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	2,082	1,341
Due in 1 Year or Less, Yield	0.64%
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	0.64%
Amortized cost	2,081	1,341
Debt instruments [Member] | Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	209
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	209	633
Due in 1 Year or Less, Yield	0.19%
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	0.19%
Amortized cost	209	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	5
Due after 1 Year through 5 Years, Amount	48
Due after 5 Years through 10 Years, Amount	37
Due after 10 Years, Amount	3
Total, fair value	93	107
Due in 1 Year or Less, Yield	5.80%
Due after 1 Year through 5 Years, Yield	4.38%
Due after 5 Years through 10 Years, Yield	5.15%
Due after 10 Years, Yield	6.71%
Total, Yield	4.76%
Amortized cost	86	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	91
Total, fair value	96	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	4.17%
Due after 10 Years, Yield	3.53%
Total, Yield	3.58%
Amortized cost	91	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	7.63%
Total, Yield	7.63%
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	17
Total, fair value	23	26
Due in 1 Year or Less, Yield	0.00%
Due after 1 Year through 5 Years, Yield	3.51%
Due after 5 Years through 10 Years, Yield	1.24%
Due after 10 Years, Yield	3.97%
Total, Yield	3.72%
Amortized cost	22	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	4
Total, fair value	8	13
Due in 1 Year or Less, Yield	0.00%
Due after 1 Year through 5 Years, Yield	3.11%
Due after 5 Years through 10 Years, Yield	1.04%
Due after 10 Years, Yield	1.62%
Total, Yield	1.67%
Amortized cost	8	13
Equity instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	2,414
Total, Yield	6.01%
Equity instruments [Member] | Fixed income mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	1,955
Total, Yield	6.49%
Equity instruments [Member] | Equity mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	459	451
Total, Yield	3.95%
Amortized cost	$ 270	$ 268