Investments in Marketable Securities (AFS Securities) (Details) (USD $)	12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 5,065,000,000	$ 5,057,000,000
Unrealized gains	333,000,000	355,000,000
Unrealized losses	(9,000,000)	(15,000,000)
Total, fair value	5,389,000,000	5,397,000,000
Investments in affiliates' commercial paper	0	2,000,000
90-day share redemption time period	90 days
Private placement share redemption percentage	1.00%
Private placement share redemption amount	250,000
Fixed income mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	1,800,000,000	2,000,000,000
Debt instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	3,073,000,000
Debt instruments [Member] | U.S. government and agency obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	652,000,000	101,000,000
Unrealized gains	1,000,000	3,000,000
Unrealized losses	(1,000,000)	0
Total, fair value	652,000,000	104,000,000
Debt instruments [Member] | Municipal debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	10,000,000	14,000,000
Unrealized gains	1,000,000	2,000,000
Unrealized losses	0	0
Total, fair value	11,000,000	16,000,000
Debt instruments [Member] | Certificates Of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	1,599,000,000	2,040,000,000
Unrealized gains	0	1,000,000
Unrealized losses	0	0
Total, fair value	1,599,000,000	2,041,000,000
Debt instruments [Member] | Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	507,000,000	495,000,000
Unrealized gains	0	0
Unrealized losses	0	0
Total, fair value	507,000,000	495,000,000
Debt instruments [Member] | Foreign government debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	0	3,000,000
Unrealized gains	0	0
Unrealized losses	0	0
Total, fair value	0	3,000,000
Debt instruments [Member] | Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	164,000,000	122,000,000
Unrealized gains	6,000,000	6,000,000
Unrealized losses	(1,000,000)	0
Total, fair value	169,000,000	128,000,000
Debt instruments [Member] | U.S. government agency mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	60,000,000	83,000,000
Unrealized gains	1,000,000	4,000,000
Unrealized losses	(1,000,000)	0
Total, fair value	60,000,000	87,000,000
Debt instruments [Member] | Non-agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	4,000,000	4,000,000
Unrealized gains	1,000,000	1,000,000
Unrealized losses	0	0
Total, fair value	5,000,000	5,000,000
Debt instruments [Member] | Non-agency commercial mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	44,000,000	50,000,000
Unrealized gains	1,000,000	1,000,000
Unrealized losses	(2,000,000)	0
Total, fair value	43,000,000	51,000,000
Debt instruments [Member] | Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	27,000,000	13,000,000
Unrealized gains	0	0
Unrealized losses	0	0
Total, fair value	27,000,000	13,000,000
Equity instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	2,316,000,000
Equity instruments [Member] | Fixed income mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	1,835,000,000
Equity instruments [Member] | Fixed income short-term sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	41,000,000	40,000,000
Unrealized gains	3,000,000	3,000,000
Unrealized losses	0	0
Total, fair value	44,000,000	43,000,000
Equity instruments [Member] | Fixed income U.S. government sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	329,000,000	296,000,000
Unrealized gains	0	16,000,000
Unrealized losses	(2,000,000)	0
Total, fair value	327,000,000	312,000,000
Equity instruments [Member] | Fixed income municipal sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	21,000,000	19,000,000
Unrealized gains	1,000,000	3,000,000
Unrealized losses	0	0
Total, fair value	22,000,000	22,000,000
Equity instruments [Member] | Investment Grade Corporate Sector Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	283,000,000	273,000,000
Unrealized gains	33,000,000	54,000,000
Unrealized losses	0	0
Total, fair value	316,000,000	327,000,000
Equity instruments [Member] | Fixed income high-yield sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	38,000,000	34,000,000
Unrealized gains	7,000,000	8,000,000
Unrealized losses	0	0
Total, fair value	45,000,000	42,000,000
Equity instruments [Member] | Fixed income real return sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	275,000,000	284,000,000
Unrealized gains	0	9,000,000
Unrealized losses	(1,000,000)	0
Total, fair value	274,000,000	293,000,000
Equity instruments [Member] | Fixed income mortgage sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	519,000,000	663,000,000
Unrealized gains	1,000,000	0
Unrealized losses	0	(15,000,000)
Total, fair value	520,000,000	648,000,000
Equity instruments [Member] | Fixed income asset-backed securities sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	40,000,000	38,000,000
Unrealized gains	10,000,000	9,000,000
Unrealized losses	0	0
Total, fair value	50,000,000	47,000,000
Equity instruments [Member] | Fixed income emerging market sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	65,000,000	63,000,000
Unrealized gains	1,000,000	3,000,000
Unrealized losses	0	0
Total, fair value	66,000,000	66,000,000
Equity instruments [Member] | Fixed income international sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	170,000,000	163,000,000
Unrealized gains	2,000,000	7,000,000
Unrealized losses	(1,000,000)	0
Total, fair value	171,000,000	170,000,000
Equity instruments [Member] | Equity mutual fund - S&P 500 index [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	217,000,000	259,000,000
Unrealized gains	264,000,000	225,000,000
Unrealized losses	0	0
Total, fair value	$ 481,000,000	$ 484,000,000