Investments in Marketable Securities (Naratives) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Mar. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	$ 2,560
Due after 1 Year through 5 Years, Amount	104
Due after 5 Years through 10 Years, Amount	113
Due after 10 Years, Amount	166
Total, fair value	5,397	4,659
Due in 1 Year or Less, Yield	0.50%
Due after 1 Year through 5 Years, Yield	3.09%
Due after 5 Years through 10 Years, Yield	3.41%
Due after 10 Years, Yield	3.99%
Total, Yield	2.75%
Due in 1 Year or Less, Amortized Cost	2,559
Due after 1 Year through 5 Years, Amortized Cost	100
Due after 5 Years through 10 Years, Amortized Cost	107
Due after 10 Years, Amortized Cost	160
Amortized cost	5,057	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	2,560
Due after 1 Year through 5 Years, Amount	104
Due after 5 Years through 10 Years, Amount	113
Due after 10 Years, Amount	166
Total, fair value	2,943
Due in 1 Year or Less, Yield	0.50%
Due after 1 Year through 5 Years, Yield	3.09%
Due after 5 Years through 10 Years, Yield	3.41%
Due after 10 Years, Yield	3.99%
Total, Yield	0.89%
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	46
Due after 5 Years through 10 Years, Amount	41
Due after 10 Years, Amount	5
Total, fair value	104	108
Due in 1 Year or Less, Yield	0.11%
Due after 1 Year through 5 Years, Yield	1.44%
Due after 5 Years through 10 Years, Yield	1.78%
Due after 10 Years, Yield	1.84%
Total, Yield	1.52%
Amortized cost	101	108
Debt instruments [Member] | Municipal 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	0
Due after 5 Years through 10 Years, Amount	2
Due after 10 Years, Amount	12
Total, fair value	16	20
Due in 1 Year or Less, Yield	1.33%
Due after 1 Year through 5 Years, Yield	0.00%
Due after 5 Years through 10 Years, Yield	5.54%
Due after 10 Years, Yield	5.76%
Total, Yield	5.33%
Amortized cost	14	17
Debt instruments [Member] | Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	2,041
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	2,041	1,341
Due in 1 Year or Less, Yield	0.56%
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	0.56%
Amortized cost	2,040	1,341
Debt instruments [Member] | Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	495
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	495	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	495	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	10
Due after 1 Year through 5 Years, Amount	51
Due after 5 Years through 10 Years, Amount	56
Due after 10 Years, Amount	11
Total, fair value	128	107
Due in 1 Year or Less, Yield	4.75%
Due after 1 Year through 5 Years, Yield	4.52%
Due after 5 Years through 10 Years, Yield	4.80%
Due after 10 Years, Yield	6.09%
Total, Yield	4.60%
Amortized cost	122	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	82
Total, fair value	87	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	4.11%
Due after 10 Years, Yield	3.49%
Total, Yield	3.53%
Amortized cost	83	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	5
Total, fair value	5	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	10.92%
Total, Yield	10.92%
Amortized cost	4	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	45
Total, fair value	51	26
Due in 1 Year or Less, Yield	0.00%
Due after 1 Year through 5 Years, Yield	3.56%
Due after 5 Years through 10 Years, Yield	1.23%
Due after 10 Years, Yield	3.65%
Total, Yield	3.44%
Amortized cost	50	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	0
Due after 5 Years through 10 Years, Amount	7
Due after 10 Years, Amount	6
Total, fair value	13	13
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	1.10%
Due after 10 Years, Yield	1.68%
Total, Yield	1.42%
Amortized cost	13	13
Equity instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	2,454
Total, Yield	4.99%
Equity instruments [Member] | Fixed income mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	1,970
Total, Yield	5.27%
Equity instruments [Member] | Equity mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	484	451
Total, Yield	3.85%
Amortized cost	$ 259	$ 268