Investments in Marketable Securities (Realized and Unrealized Losses) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Mar. 31, 2013
OTTI losses recognized in income
Impairment write-downs	$ 23	$ 0	$ 53	$ 0
Aging of fair value and gross unrealized losses for AFS securities
Less than 12 months, fair value	151		151		532
12 months or more, fair value	0		0		116
Less than 12 months, unrealized losses	(8)		(8)		(12)
12 months or more, unrealized losses	0		0		(3)
Corporate debt securities [Member] | Debt instruments [Member]
Aging of fair value and gross unrealized losses for AFS securities
Less than 12 months, fair value	51		51		0
Less than 12 months, unrealized losses	(3)		(3)		0
U.S. government and agency obligations [Member] | Debt instruments [Member]
Aging of fair value and gross unrealized losses for AFS securities
Less than 12 months, fair value	28		28		0
Less than 12 months, unrealized losses	(1)		(1)		0
U.S. government agency mortgage-backed securities [Member] | Debt instruments [Member]
Aging of fair value and gross unrealized losses for AFS securities
Less than 12 months, fair value	29		29		0
Less than 12 months, unrealized losses	(1)		(1)		0
U.S. government sector fund [Member] | Equity instruments [Member]
Aging of fair value and gross unrealized losses for AFS securities
Less than 12 months, fair value	0		0		0
Less than 12 months, unrealized losses	0		0		0
Short-term sector fund [Member] | Equity instruments [Member]
Aging of fair value and gross unrealized losses for AFS securities
Less than 12 months, fair value					0
Less than 12 months, unrealized losses	0		0		0
Real return sector fund [Member] | Equity instruments [Member]
Aging of fair value and gross unrealized losses for AFS securities
Less than 12 months, fair value	0		0		0
Less than 12 months, unrealized losses	0		0		0
International sector fund [Member] | Equity instruments [Member]
Aging of fair value and gross unrealized losses for AFS securities
Less than 12 months, fair value	0		0		0
Less than 12 months, unrealized losses	0		0		0
Mortgage sector fund [Member] | Equity instruments [Member]
Aging of fair value and gross unrealized losses for AFS securities
Less than 12 months, fair value	0		0		532
Less than 12 months, unrealized losses	0		0		(12)
Non-agency commercial mortgage-backed securities [Member] | Debt instruments [Member]
Aging of fair value and gross unrealized losses for AFS securities
Less than 12 months, fair value	27		27
Less than 12 months, unrealized losses	(2)		(2)
Emerging market sector fund [Member] | Equity instruments [Member]
Aging of fair value and gross unrealized losses for AFS securities
Less than 12 months, fair value	0		0		0
Less than 12 months, unrealized losses	0		0		0
High-yield sector fund [Member] | Equity instruments [Member]
Aging of fair value and gross unrealized losses for AFS securities
Less than 12 months, fair value	16		16		0
Less than 12 months, unrealized losses	$ (1)		$ (1)		$ 0