Income Per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Basic income per common share
Net income	$ 240	$ 222	$ 575	$ 462
Less: Net income allocable to participating securities	(2)	(2)	(6)	(5)
Net income allocable to common shares	238	220	569	457
Weighted average common shares outstanding	448	458	449	462
Basic income per common share	$ 0.53	$ 0.48	$ 1.27	$ 0.99
Diluted income per common share
Net income	240	222	575	462
Less: Net income allocable to participating securities	(2)	(2)	(6)	(5)
Net income allocable to common shares	$ 238	$ 220	$ 569	$ 457
Weighted average shares outstanding and common share equivalents
Weighted average common shares outstanding	448	458	449	462
Weighted average effect of share-based payment awards	2	1	1	1
Denominator in calculation of diluted income per share	450	459	450	463
Diluted income per common share	$ 0.53	$ 0.48	$ 1.26	$ 0.99
Income Per Common Share (Textual) [Abstract]
Number of anti-dilutive restricted stock awards and options excluded from the calculation	2	4	2	3
Weighted average restricted stock awards considered participating securities	5	5	5	6