Earnings Per Common Share (Computations for Earnings Per Common Share) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended				6 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net Income	$ 401		$ 296		$ 781		$ 582
Weighted average common shares outstanding:
Basic	236		246		236		247
Diluted	240		250		240		252
Earnings Per Common Share
Diluted	$ 1.67	[1]	$ 1.18	[1]	$ 3.25	[1]	$ 2.31	[1]
Basic	$ 1.70	[1]	$ 1.20	[1]	$ 3.31	[1]	$ 2.35	[1]
Options [Member]
Effect of dilutive securities:
Dilutive securities	1		1		1		2
Restricted Stock Units [Member]
Effect of dilutive securities:
Dilutive securities	3		3		3		3

[1] Certain computations may reflect rounding adjustments