Income Per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Basic income per common share
Net income	$ 335	$ 240
Less: Net income allocable to participating securities	(4)	(3)
Net income allocable to common shares	331	237
Weighted average common shares outstanding	450	465
Basic income per common share	$ 0.74	$ 0.51
Diluted income per common share
Net income	335	240
Less: Net income allocable to participating securities	(4)	(3)
Net income allocable to common shares	$ 331	$ 237
Weighted average shares outstanding and common share equivalents
Weighted average common shares outstanding	450	465
Weighted average effect of share-based payment awards	1	2
Denominator in calculation of diluted income per share	451	467
Diluted income per common share	$ 0.73	$ 0.51
Income Per Common Share (Textual) [Abstract]
Number of anti-dilutive restricted stock awards and options excluded from the calculation	4	3
Weighted average restricted stock awards considered participating securities	5	6