Income Taxes (Gross Unrecognized Tax Benefits) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011
Income Tax Disclosure [Abstract]
Unrecognized tax benefits at period start	$ 595	$ 635	$ 619
Additions based on tax positions related to prior years	46	11	32
Reductions based on tax positions related to prior years	(108)	(72)	(60)
Additions based on tax positions related to current year	31	37	50
Reductions based on settlements	(2)	(1)	(6)
Reductions based on the lapse of the applicable statutes of limitations	(2)	(15)	0
Unrecognized tax benefits at period end	$ 560	$ 595	$ 635