Investments in Marketable Securities (AFS Securities) (Details) (USD $)	3 Months Ended
	Jun. 30, 2013	Mar. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 4,676,000,000	$ 5,057,000,000
Unrealized gains	312,000,000	355,000,000
Unrealized losses	(31,000,000)	(15,000,000)
Total, fair value	4,957,000,000	5,397,000,000
Investments in affiliates commercial paper	24,000,000	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	1,900,000,000	2,000,000,000
Debt instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	2,588,000,000
Debt instruments [Member] | U.S. government and agency obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	78,000,000	101,000,000
Unrealized gains	1,000,000	3,000,000
Unrealized losses	(1,000,000)	0
Total, fair value	78,000,000	104,000,000
Debt instruments [Member] | Municipal debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	14,000,000	14,000,000
Unrealized gains	1,000,000	2,000,000
Unrealized losses	0	0
Total, fair value	15,000,000	16,000,000
Debt instruments [Member] | Certificates Of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	1,836,000,000	2,040,000,000
Unrealized gains	1,000,000	1,000,000
Unrealized losses	0	0
Total, fair value	1,837,000,000	2,041,000,000
Debt instruments [Member] | Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	355,000,000	495,000,000
Unrealized gains	0	0
Unrealized losses	0	0
Total, fair value	355,000,000	495,000,000
Debt instruments [Member] | Foreign government debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	3,000,000	3,000,000
Unrealized gains	0	0
Unrealized losses	0	0
Total, fair value	3,000,000	3,000,000
Debt instruments [Member] | Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	149,000,000	122,000,000
Unrealized gains	4,000,000	6,000,000
Unrealized losses	(4,000,000)	0
Total, fair value	149,000,000	128,000,000
Debt instruments [Member] | U.S. government agency mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	78,000,000	83,000,000
Unrealized gains	2,000,000	4,000,000
Unrealized losses	(1,000,000)	0
Total, fair value	79,000,000	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	49,000,000	50,000,000
Unrealized gains	1,000,000	1,000,000
Unrealized losses	(2,000,000)	0
Total, fair value	48,000,000	51,000,000
Debt instruments [Member] | Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	19,000,000	13,000,000
Unrealized gains	0	0
Unrealized losses	0	0
Total, fair value	19,000,000	13,000,000
Equity instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	2,369,000,000
Equity instruments [Member] | Fixed income mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	1,874,000,000
Equity instruments [Member] | Fixed income short-term sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	40,000,000	40,000,000
Unrealized gains	3,000,000	3,000,000
Unrealized losses	0	0
Total, fair value	43,000,000	43,000,000
Equity instruments [Member] | Fixed income U.S. government sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	324,000,000	296,000,000
Unrealized gains	0	16,000,000
Unrealized losses	(5,000,000)	0
Total, fair value	319,000,000	312,000,000
Equity instruments [Member] | Fixed income municipal sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	20,000,000	19,000,000
Unrealized gains	1,000,000	3,000,000
Unrealized losses	0	0
Total, fair value	21,000,000	22,000,000
Equity instruments [Member] | Fixed income investment grade corporate sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	259,000,000	273,000,000
Unrealized gains	41,000,000	54,000,000
Unrealized losses	0	0
Total, fair value	300,000,000	327,000,000
Equity instruments [Member] | Fixed income high-yield sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	35,000,000	34,000,000
Unrealized gains	6,000,000	8,000,000
Unrealized losses	0	0
Total, fair value	41,000,000	42,000,000
Equity instruments [Member] | Fixed income real return sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	286,000,000	284,000,000
Unrealized gains	0	9,000,000
Unrealized losses	(16,000,000)	0
Total, fair value	270,000,000	293,000,000
Equity instruments [Member] | Fixed income mortgage sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	599,000,000	663,000,000
Unrealized gains	0	0
Unrealized losses	0	(15,000,000)
Total, fair value	599,000,000	648,000,000
Equity instruments [Member] | Fixed income asset-backed securities sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	39,000,000	38,000,000
Unrealized gains	9,000,000	9,000,000
Unrealized losses	0	0
Total, fair value	48,000,000	47,000,000
Equity instruments [Member] | Fixed income emerging market sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	64,000,000	63,000,000
Unrealized gains	1,000,000	3,000,000
Unrealized losses	(2,000,000)	0
Total, fair value	63,000,000	66,000,000
Equity instruments [Member] | Fixed income international sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	166,000,000	163,000,000
Unrealized gains	4,000,000	7,000,000
Unrealized losses	0	0
Total, fair value	170,000,000	170,000,000
Equity instruments [Member] | Equity mutual fund - S&P 500 index [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	259,000,000	259,000,000
Unrealized gains	236,000,000	225,000,000
Unrealized losses	0	0
Total, fair value	$ 495,000,000	$ 484,000,000