Income Per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013	Dec. 31, 2012
Basic income per common share
Net income	$ 232	$ 251	$ 807	$ 713
Less: Net income allocable to participating securities	(2)	(3)	(8)	(8)
Net income allocable to common shares	230	248	799	705
Weighted average common shares outstanding	446	452	448	458
Basic income per common share	$ 0.52	$ 0.55	$ 1.78	$ 1.54
Diluted income per common share
Net income	232	251	807	713
Less: Net income allocable to participating securities	(2)	(3)	(8)	(8)
Net income allocable to common shares	$ 230	$ 248	$ 799	$ 705
Weighted average shares outstanding and common share equivalents
Weighted average common shares outstanding	446	452	448	458
Weighted average effect of share-based payment awards	2	1	1	2
Denominator in calculation of diluted income per share	448	453	449	460
Diluted income per common share	$ 0.51	$ 0.55	$ 1.78	$ 1.53
Income Per Common Share (Textual) [Abstract]
Number of anti-dilutive restricted stock awards and options excluded from the calculation	1	4	2	4
Weighted average restricted stock awards considered participating securities	5	5	5	6