Investments in Marketable Securities (Naratives) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Mar. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	$ 1,634
Due after 1 Year through 5 Years, Amount	120
Due after 5 Years through 10 Years, Amount	98
Due after 10 Years, Amount	168
Total, fair value	4,364	5,397
Due in 1 Year or Less, Yield	0.43%
Due after 1 Year through 5 Years, Yield	2.61%
Due after 5 Years through 10 Years, Yield	3.01%
Due after 10 Years, Yield	3.61%
Total, Yield	2.40%
Due in 1 Year or Less, Amortized Cost	1,633
Due after 1 Year through 5 Years, Amortized Cost	117
Due after 5 Years through 10 Years, Amortized Cost	100
Due after 10 Years, Amortized Cost	166
Amortized cost	4,043	5,057
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	1,800	2,000
Debt instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	1,634
Due after 1 Year through 5 Years, Amount	120
Due after 5 Years through 10 Years, Amount	98
Due after 10 Years, Amount	168
Total, fair value	2,020
Due in 1 Year or Less, Yield	0.43%
Due after 1 Year through 5 Years, Yield	2.61%
Due after 5 Years through 10 Years, Yield	3.01%
Due after 10 Years, Yield	3.61%
Total, Yield	0.95%
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	7
Due after 1 Year through 5 Years, Amount	71
Due after 5 Years through 10 Years, Amount	26
Due after 10 Years, Amount	1
Total, fair value	105	104
Due in 1 Year or Less, Yield	0.20%
Due after 1 Year through 5 Years, Yield	1.55%
Due after 5 Years through 10 Years, Yield	1.56%
Due after 10 Years, Yield	2.80%
Total, Yield	1.43%
Amortized cost	104	101
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	1
Due after 10 Years, Amount	11
Total, fair value	14	16
Due in 1 Year or Less, Yield	2.39%
Due after 1 Year through 5 Years, Yield	0.00%
Due after 5 Years through 10 Years, Yield	5.57%
Due after 10 Years, Yield	5.69%
Total, Yield	5.18%
Amortized cost	14	14
Debt instruments [Member] | Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	1,412
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	1,412	2,041
Due in 1 Year or Less, Yield	0.43%
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	0.43%
Amortized cost	1,411	2,040
Debt instruments [Member] | Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	202
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	202	495
Due in 1 Year or Less, Yield	0.27%
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	0.27%
Amortized cost	202	495
Debt instruments [Member] | Foreign government debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	3
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	3	3
Due in 1 Year or Less, Yield	2.93%
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	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	8
Due after 1 Year through 5 Years, Amount	40
Due after 5 Years through 10 Years, Amount	62
Due after 10 Years, Amount	27
Total, fair value	137	128
Due in 1 Year or Less, Yield	4.05%
Due after 1 Year through 5 Years, Yield	4.72%
Due after 5 Years through 10 Years, Yield	3.52%
Due after 10 Years, Yield	3.66%
Total, Yield	3.95%
Amortized cost	136	122
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	4
Due after 10 Years, Amount	70
Total, fair value	74	87
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	3.94%
Due after 10 Years, Yield	3.01%
Total, Yield	3.07%
Amortized cost	73	83
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	5
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	11.23%
Total, Yield	11.23%
Amortized cost	4	4
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	1
Due after 10 Years, Amount	44
Total, fair value	49	51
Due in 1 Year or Less, Yield	0.00%
Due after 1 Year through 5 Years, Yield	1.70%
Due after 5 Years through 10 Years, Yield	4.24%
Due after 10 Years, Yield	3.41%
Total, Yield	3.31%
Amortized cost	50	50
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	5
Due after 5 Years through 10 Years, Amount	4
Due after 10 Years, Amount	10
Total, fair value	19	13
Due in 1 Year or Less, Yield	0.00%
Due after 1 Year through 5 Years, Yield	0.74%
Due after 5 Years through 10 Years, Yield	1.83%
Due after 10 Years, Yield	2.68%
Total, Yield	1.97%
Amortized cost	19	13
Equity instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	2,344
Total, Yield	3.65%
Equity instruments [Member] | Fixed income mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	1,845
Total, Yield	3.53%
Equity instruments [Member] | Equity mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	499	484
Total, Yield	4.10%
Amortized cost	$ 250	$ 259