Investments In Marketable Securities (AFS Securities) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Mar. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 4,563	$ 4,403
Unrealized gains	283	275
Unrealized losses	(10)	(19)
Total, fair value	4,836	4,659
Commercial paper issued by affiliated entity	0	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	2,464
Debt instruments [Member] | U.S. government and agency obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	131	108
Unrealized gains	4	1
Unrealized losses	0	(1)
Total, fair value	135	108
Debt instruments [Member] | Municipal debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	16	17
Unrealized gains	3	3
Unrealized losses	0	0
Total, fair value	19	20
Debt instruments [Member] | Certificates Of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	1,724	1,341
Unrealized gains	0	0
Unrealized losses	0	0
Total, fair value	1,724	1,341
Debt instruments [Member] | Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	344	633
Unrealized gains	0	0
Unrealized losses	0	0
Total, fair value	344	633
Debt instruments [Member] | Foreign government debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost		3
Unrealized gains		0
Unrealized losses		0
Total, fair value	3	3
Debt instruments [Member] | Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	89	100
Unrealized gains	6	7
Unrealized losses	0	0
Total, fair value	95	107
Debt instruments [Member] | U.S. government agency mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	94	100
Unrealized gains	6	5
Unrealized losses	0	0
Total, fair value	100	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	24	25
Unrealized gains	1	1
Unrealized losses	0	0
Total, fair value	25	26
Debt instruments [Member] | Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	12	13
Unrealized gains	0	0
Unrealized losses	0	0
Total, fair value	12	13
Equity instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	2,372
Equity instruments [Member] | Fixed income mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	1,933
Equity instruments [Member] | Fixed income short-term sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	39	39
Unrealized gains	1	1
Unrealized losses	0	0
Total, fair value	40	40
Equity instruments [Member] | Fixed income U.S. government sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	330	319
Unrealized gains	12	0
Unrealized losses	0	(6)
Total, fair value	342	313
Equity instruments [Member] | Fixed income municipal sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	19	19
Unrealized gains	2	2
Unrealized losses	0	0
Total, fair value	21	21
Equity instruments [Member] | Fixed income investement grade corporate sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	260	261
Unrealized gains	39	37
Unrealized losses	0	0
Total, fair value	299	298
Equity instruments [Member] | Fixed income high-yield sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	31	31
Unrealized gains	6	6
Unrealized losses	0	0
Total, fair value	37	37
Equity instruments [Member] | Fixed income real return sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	279	228
Unrealized gains	8	3
Unrealized losses	0	0
Total, fair value	287	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	(10)	(12)
Total, fair value	637	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	5	4
Unrealized losses	0	0
Total, fair value	42	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	4	2
Unrealized losses	0	0
Total, fair value	64	62
Equity instruments [Member] | Fixed income international sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	147	143
Unrealized gains	17	19
Unrealized losses	0	0
Total, fair value	164	162
Equity instruments [Member] | Equity mutual fund - S&P 500 index [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	271	268
Unrealized gains	168	183
Unrealized losses	0	0
Total, fair value	$ 439	$ 451