Share-Based Compensation (RSU Activity) (Details) (Restricted Stock Units (RSUs) [Member], USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested, Shares, Beginning Balance	7	6	4
Granted, Shares	1	2	3
Vested, Shares	(2)	(1)	(1)
Nonvested, Shares, Ending Balance	6	7	6
Nonvested, Weighted Average Grant Date Fair Value Per Share, Beginning of Period	$ 65.14	$ 57.79	$ 49.21
Granted, Weighted-Average Grant Date Fair Value Per Share	$ 87.86	$ 82.71	$ 67.84
Vested, Weighted Average Grant Date Fair Value Per Share	$ 41.80	$ 57.95	$ 61.05
Nonvested, Weighted Average Grant Date Fair Value Per Share, End of Period	$ 76.20	$ 65.14	$ 57.79