Share-Based Compensation (RSU Activity) (Details) (RSUs [Member], USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2010
RSUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested, Shares, Beginning balance	6	4	3
Granted, Shares	2	3	2
Vested, Shares	(1)	(1)	(1)
Nonvested, Shares, Ending balance	7	6	4
Nonvested, Weighted-Average Grant Date Fair Value Per Share, Beginning of period	$ 57.79	$ 49.21	$ 54.70
Granted, Weighted-Average Grant Date Fair Value Per Share	$ 82.71	$ 67.84	$ 40.94
Vested, Weighted-Average Grant Date Fair Value Per Share	$ 57.95	$ 61.05	$ 50.42
Nonvested, Weighted-Average Grant Date Fair Value Per Share, End of period	$ 65.14	$ 57.79	$ 49.21