Investments in Marketable Securities (Realized and Unrealized Losses) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Mar. 31, 2012
Aging of fair value and gross unrealized losses for AFS securities
Less than 12 months, fair value	$ 637	$ 944
Less than 12 months, unrealized losses	(10)	(15)
Corporate debt 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
Commercial paper [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 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]
Aging of fair value and gross unrealized losses for AFS securities
Less than 12 months, unrealized losses	0	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	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)
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	0
Less than 12 months, unrealized losses	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
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	637	639
Less than 12 months, unrealized losses	(10)	(12)
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
Less than 12 months, unrealized losses	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
Less than 12 months, unrealized losses	$ 0	$ 0