Earnings Per Common Share (Basic and Diluted Earnings per Common Share) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended				9 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2012		Dec. 31, 2011
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net Income	$ 298		$ 300		$ 1,079		$ 882
Weighted average common shares outstanding:
Basic	235		246		236		247
Diluted	240		251		240		252
Earnings Per Common Share
Diluted	$ 1.24	[1]	$ 1.20	[1]	$ 4.49	[1]	$ 3.51	[1]
Basic	$ 1.27	[1]	$ 1.22	[1]	$ 4.58	[1]	$ 3.57	[1]
Options [Member]
Effect of dilutive securities:
Dilutive securities	1		2		1		2
Restricted Stock Units [Member]
Effect of dilutive securities:
Dilutive securities	4		3		3		3

[1] Certain computations may reflect rounding adjustments