Investments In Marketable Securities (AFS Securities) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Mar. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 4,824	$ 4,403
Unrealized gains	338	275
Unrealized losses	(2)	(19)
Total, fair value	5,160	4,659
Commercial paper issued by affiliated entity	5	0
Fixed income mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	1,900	1,800
Debt instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	2,759
Debt instruments [Member] | U.S. government and agency obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	138	108
Unrealized gains	4	1
Unrealized losses	0	(1)
Total, fair value	142	108
Debt instruments [Member] | Municipal debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	18	17
Unrealized gains	4	3
Unrealized losses	0	0
Total, fair value	22	20
Debt instruments [Member] | Certificates Of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	1,844	1,341
Unrealized gains	1	0
Unrealized losses	0	0
Total, fair value	1,845	1,341
Debt instruments [Member] | Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	511	633
Unrealized gains	0	0
Unrealized losses	0	0
Total, fair value	511	633
Debt instruments [Member] | Foreign government debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	3	3
Unrealized gains	0	0
Unrealized losses	0	0
Total, fair value	3	3
Debt instruments [Member] | Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	85	100
Unrealized gains	7	7
Unrealized losses	0	0
Total, fair value	92	107
Debt instruments [Member] | U.S. government agency mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	98	100
Unrealized gains	6	5
Unrealized losses	0	0
Total, fair value	104	105
Debt instruments [Member] | Non-agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	6	7
Unrealized gains	1	1
Unrealized losses	0	0
Total, fair value	7	8
Debt instruments [Member] | Non-agency commercial mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	23	25
Unrealized gains	1	1
Unrealized losses	0	0
Total, fair value	24	26
Debt instruments [Member] | Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	9	13
Unrealized gains	0	0
Unrealized losses	0	0
Total, fair value	9	13
Equity instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	2,401
Equity instruments [Member] | Fixed income mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	1,935
Equity instruments [Member] | Fixed income short-term sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	38	39
Unrealized gains	3	1
Unrealized losses	0	0
Total, fair value	41	40
Equity instruments [Member] | Fixed income U.S. government sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	293	319
Unrealized gains	16	0
Unrealized losses	0	(6)
Total, fair value	309	313
Equity instruments [Member] | Fixed income municipal sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	19	19
Unrealized gains	3	2
Unrealized losses	0	0
Total, fair value	22	21
Equity instruments [Member] | Fixed income investement grade corporate sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	262	261
Unrealized gains	52	37
Unrealized losses	0	0
Total, fair value	314	298
Equity instruments [Member] | Fixed income high-yield sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	32	31
Unrealized gains	7	6
Unrealized losses	0	0
Total, fair value	39	37
Equity instruments [Member] | Fixed income real return sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	276	228
Unrealized gains	14	3
Unrealized losses	0	0
Total, fair value	290	231
Equity instruments [Member] | Fixed income mortgage sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	647	651
Unrealized gains	0	0
Unrealized losses	(2)	(12)
Total, fair value	645	639
Equity instruments [Member] | Fixed income asset-backed securities sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	37	37
Unrealized gains	7	4
Unrealized losses	0	0
Total, fair value	44	41
Equity instruments [Member] | Fixed income emerging market sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	60	60
Unrealized gains	6	2
Unrealized losses	0	0
Total, fair value	66	62
Equity instruments [Member] | Fixed income international sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	152	143
Unrealized gains	13	19
Unrealized losses	0	0
Total, fair value	165	162
Equity instruments [Member] | Equity mutual fund - S&P 500 index [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	273	268
Unrealized gains	193	183
Unrealized losses	0	0
Total, fair value	$ 466	$ 451