Investments in Marketable Securities (Realized and Unrealized Losses) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
OTTI losses recognized in income
Total other-than-temporary impairment losses	$ 0	$ 0
Aging of fair value and gross unrealized losses for AFS securities
Less than 12 months, fair value	532	944
12 months or more, fair value	116
Less than 12 months, unrealized losses	(12)	(15)
12 months or more, unrealized losses	(3)
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	0	68
Less than 12 months, unrealized losses	0	(1)
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	0	0
Less than 12 months, unrealized losses	0	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	237
Less than 12 months, unrealized losses	0	(2)
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
Less than 12 months, unrealized losses	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	532	639
Less than 12 months, unrealized losses	(12)	(12)
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
Less than 12 months, unrealized losses	$ 0	$ 0