Investments in Marketable Securities (Naratives) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Mar. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	$ 2,208
Due after 1 Year through 5 Years, Amount	84
Due after 5 Years through 10 Years, Amount	107
Due after 10 Years, Amount	189
Total, fair value	4,957	5,397
Due in 1 Year or Less, Yield	0.40%
Due after 1 Year through 5 Years, Yield	3.08%
Due after 5 Years through 10 Years, Yield	2.96%
Due after 10 Years, Yield	3.47%
Total, Yield	2.28%
Due in 1 Year or Less, Amortized Cost	2,206
Due after 1 Year through 5 Years, Amortized Cost	83
Due after 5 Years through 10 Years, Amortized Cost	109
Due after 10 Years, Amortized Cost	187
Amortized cost	4,676	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	1,900	2,000
Debt instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	2,208
Due after 1 Year through 5 Years, Amount	84
Due after 5 Years through 10 Years, Amount	107
Due after 10 Years, Amount	189
Total, fair value	2,588
Due in 1 Year or Less, Yield	0.40%
Due after 1 Year through 5 Years, Yield	3.08%
Due after 5 Years through 10 Years, Yield	2.96%
Due after 10 Years, Yield	3.47%
Total, Yield	0.82%
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	8
Due after 1 Year through 5 Years, Amount	42
Due after 5 Years through 10 Years, Amount	23
Due after 10 Years, Amount	5
Total, fair value	78	104
Due in 1 Year or Less, Yield	0.19%
Due after 1 Year through 5 Years, Yield	1.50%
Due after 5 Years through 10 Years, Yield	1.66%
Due after 10 Years, Yield	3.16%
Total, Yield	1.52%
Amortized cost	78	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	12
Total, fair value	15	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	5.17%
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	1,837
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	1,837	2,041
Due in 1 Year or Less, Yield	0.44%
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	0.44%
Amortized cost	1,836	2,040
Debt instruments [Member] | Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in 1 Year or Less, Amount	355
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	355	495
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	355	495
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	6
Due after 1 Year through 5 Years, Amount	36
Due after 5 Years through 10 Years, Amount	68
Due after 10 Years, Amount	39
Total, fair value	149	128
Due in 1 Year or Less, Yield	3.48%
Due after 1 Year through 5 Years, Yield	4.86%
Due after 5 Years through 10 Years, Yield	3.53%
Due after 10 Years, Yield	3.23%
Total, Yield	3.79%
Amortized cost	149	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	75
Total, fair value	79	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	3.92%
Due after 10 Years, Yield	2.99%
Total, Yield	3.05%
Amortized cost	78	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	10.84%
Total, Yield	10.84%
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	3
Due after 5 Years through 10 Years, Amount	2
Due after 10 Years, Amount	43
Total, fair value	48	51
Due in 1 Year or Less, Yield	0.00%
Due after 1 Year through 5 Years, Yield	3.71%
Due after 5 Years through 10 Years, Yield	1.18%
Due after 10 Years, Yield	3.35%
Total, Yield	3.30%
Amortized cost	49	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	0
Due after 5 Years through 10 Years, Amount	9
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	0.00%
Due after 5 Years through 10 Years, Yield	1.31%
Due after 10 Years, Yield	2.57%
Total, Yield	1.89%
Amortized cost	19	13
Equity instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	2,369
Total, Yield	3.86%
Equity instruments [Member] | Fixed income mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	1,874
Total, Yield	3.80%
Equity instruments [Member] | Equity mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	495	484
Total, Yield	4.09%
Amortized cost	$ 259	$ 259