Earnings Per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Jun. 30, 2012		Jun. 30, 2011
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net Income	$ 380		$ 286
Weighted average common shares outstanding:
Basic	236		249
Diluted	240		254
Earnings Per Common Share
Diluted	$ 1.58	[1]	$ 1.13	[1]
Basic	$ 1.61	[1]	$ 1.15	[1]
Potentially dilutive shares excluded from diluted earnings per share	5		4
Options [Member]
Effect of dilutive securities:
Dilutive securities	1		2
Restricted Stock Units [Member]
Effect of dilutive securities:
Dilutive securities	3		3

[1] Certain computations may reflect rounding adjustments