Investments in Marketable Securities (AFS Securities) (Details) (USD $)	Dec. 31, 2014	Mar. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 4,236,000,000	$ 5,065,000,000
Unrealized gains	353,000,000	333,000,000
Unrealized losses	(18,000,000)	(9,000,000)
Total, fair value	4,571,000,000	5,389,000,000
Private placement share redemption percentage	1.00%
Private placement share redemption amount	250,000
Debt instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,325,000,000
Total, fair value	2,335,000,000
Debt instruments [Member] | U.S. government and agency obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,010,000,000	652,000,000
Unrealized gains	1,000,000	1,000,000
Unrealized losses	(1,000,000)	(1,000,000)
Total, fair value	2,010,000,000	652,000,000
Debt instruments [Member] | Municipal debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	10,000,000	10,000,000
Unrealized gains	2,000,000	1,000,000
Unrealized losses	0	0
Total, fair value	12,000,000	11,000,000
Debt instruments [Member] | Certificates Of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost		1,599,000,000
Unrealized gains		0
Unrealized losses		0
Total, fair value		1,599,000,000
Debt instruments [Member] | Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost		507,000,000
Unrealized gains		0
Unrealized losses		0
Total, fair value		507,000,000
Debt instruments [Member] | Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	179,000,000	164,000,000
Unrealized gains	6,000,000	6,000,000
Unrealized losses	(1,000,000)	(1,000,000)
Total, fair value	184,000,000	169,000,000
Debt instruments [Member] | U.S. government agency mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	53,000,000	60,000,000
Unrealized gains	2,000,000	1,000,000
Unrealized losses	0	(1,000,000)
Total, fair value	55,000,000	60,000,000
Debt instruments [Member] | Non-agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	3,000,000	4,000,000
Unrealized gains	1,000,000	1,000,000
Unrealized losses	0	0
Total, fair value	4,000,000	5,000,000
Debt instruments [Member] | Non-agency commercial mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	45,000,000	44,000,000
Unrealized gains	1,000,000	1,000,000
Unrealized losses	(1,000,000)	(2,000,000)
Total, fair value	45,000,000	43,000,000
Debt instruments [Member] | Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	25,000,000	27,000,000
Unrealized gains		0
Unrealized losses	0	0
Total, fair value	25,000,000	27,000,000
Equity instruments [Member] | Fixed income short-term floating NAV fund II [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	190,000,000
Unrealized gains
Unrealized losses	0
Total, fair value	190,000,000
Equity instruments [Member] | Fixed income short-term sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	35,000,000	41,000,000
Unrealized gains	2,000,000	3,000,000
Unrealized losses	0	0
Total, fair value	37,000,000	44,000,000
Equity instruments [Member] | Fixed income U.S. government sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	275,000,000	329,000,000
Unrealized gains	14,000,000	0
Unrealized losses	0	(2,000,000)
Total, fair value	289,000,000	327,000,000
Equity instruments [Member] | Fixed income municipal sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	18,000,000	21,000,000
Unrealized gains	1,000,000	1,000,000
Unrealized losses	0	0
Total, fair value	19,000,000	22,000,000
Equity instruments [Member] | Investment Grade Corporate Sector Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	255,000,000	283,000,000
Unrealized gains	14,000,000	33,000,000
Unrealized losses	(4,000,000)	0
Total, fair value	265,000,000	316,000,000
Equity instruments [Member] | Fixed income high-yield sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	32,000,000	38,000,000
Unrealized gains	5,000,000	7,000,000
Unrealized losses	(1,000,000)	0
Total, fair value	36,000,000	45,000,000
Equity instruments [Member] | Fixed income real return sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	235,000,000	275,000,000
Unrealized gains		0
Unrealized losses	(8,000,000)	(1,000,000)
Total, fair value	227,000,000	274,000,000
Equity instruments [Member] | Fixed income mortgage sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	441,000,000	519,000,000
Unrealized gains	7,000,000	1,000,000
Unrealized losses	0	0
Total, fair value	448,000,000	520,000,000
Equity instruments [Member] | Fixed income asset-backed securities sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	35,000,000	40,000,000
Unrealized gains	8,000,000	10,000,000
Unrealized losses	0	0
Total, fair value	43,000,000	50,000,000
Equity instruments [Member] | Fixed income emerging market sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	57,000,000	65,000,000
Unrealized gains		1,000,000
Unrealized losses	(2,000,000)	0
Total, fair value	55,000,000	66,000,000
Equity instruments [Member] | Fixed income international sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	142,000,000	170,000,000
Unrealized gains	9,000,000	2,000,000
Unrealized losses	0	(1,000,000)
Total, fair value	151,000,000	171,000,000
Equity instruments [Member] | Equity mutual fund - S&P 500 index [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	196,000,000	217,000,000
Unrealized gains	280,000,000	264,000,000
Unrealized losses	0	0
Total, fair value	$ 476,000,000	$ 481,000,000