Investments in Marketable Securities (AFS Securities) (Details) (USD $)	3 Months Ended
	Sep. 30, 2014	Mar. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 4,733,000,000	$ 5,065,000,000
Unrealized gains	342,000,000	333,000,000
Unrealized losses	(4,000,000)	(9,000,000)
Total, fair value	5,071,000,000	5,389,000,000
Investments in affiliates' commercial paper	46,000,000	0
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	1,700,000,000	1,800,000,000
Debt instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,855,000,000
Total, fair value	2,863,000,000
Debt instruments [Member] | U.S. government and agency obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,246,000,000	652,000,000
Unrealized gains	1,000,000	1,000,000
Unrealized losses	(1,000,000)	(1,000,000)
Total, fair value	2,246,000,000	652,000,000
Debt instruments [Member] | Municipal debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	11,000,000	10,000,000
Unrealized gains	1,000,000	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	30,000,000	1,599,000,000
Unrealized gains	0	0
Unrealized losses	0	0
Total, fair value	30,000,000	1,599,000,000
Debt instruments [Member] | Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	271,000,000	507,000,000
Unrealized gains	0	0
Unrealized losses	0	0
Total, fair value	271,000,000	507,000,000
Debt instruments [Member] | Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	163,000,000	164,000,000
Unrealized gains	5,000,000	6,000,000
Unrealized losses	(1,000,000)	(1,000,000)
Total, fair value	167,000,000	169,000,000
Debt instruments [Member] | U.S. government agency mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	55,000,000	60,000,000
Unrealized gains	2,000,000	1,000,000
Unrealized losses	0	(1,000,000)
Total, fair value	57,000,000	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	46,000,000	44,000,000
Unrealized gains	1,000,000	1,000,000
Unrealized losses	(1,000,000)	(2,000,000)
Total, fair value	46,000,000	43,000,000
Debt instruments [Member] | Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	30,000,000	27,000,000
Unrealized gains	0	0
Unrealized losses	0	0
Total, fair value	30,000,000	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	0
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	271,000,000	329,000,000
Unrealized gains	3,000,000	0
Unrealized losses	0	(2,000,000)
Total, fair value	274,000,000	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	239,000,000	283,000,000
Unrealized gains	26,000,000	33,000,000
Unrealized losses	0	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	6,000,000	7,000,000
Unrealized losses	(1,000,000)	0
Total, fair value	37,000,000	45,000,000
Equity instruments [Member] | Fixed income real return sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	229,000,000	275,000,000
Unrealized gains	0	0
Unrealized losses	0	(1,000,000)
Total, fair value	229,000,000	274,000,000
Equity instruments [Member] | Fixed income mortgage sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	432,000,000	519,000,000
Unrealized gains	9,000,000	1,000,000
Unrealized losses	0	0
Total, fair value	441,000,000	520,000,000
Equity instruments [Member] | Fixed income asset-backed securities sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	33,000,000	40,000,000
Unrealized gains	9,000,000	10,000,000
Unrealized losses	0	0
Total, fair value	42,000,000	50,000,000
Equity instruments [Member] | Fixed income emerging market sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	54,000,000	65,000,000
Unrealized gains	1,000,000	1,000,000
Unrealized losses	0	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	4,000,000	2,000,000
Unrealized losses	0	(1,000,000)
Total, fair value	146,000,000	171,000,000
Equity instruments [Member] | Equity mutual fund - S&P 500 index [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	203,000,000	217,000,000
Unrealized gains	270,000,000	264,000,000
Unrealized losses	0	0
Total, fair value	$ 473,000,000	$ 481,000,000