Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Basic income per common share:
Net income	$ 178	$ 198
Less: Net income allocable to participating securities	(2)	(2)
Net income allocable to common shares	$ 176	$ 196
Weighted average common shares outstanding	415	414
Basic income per common share (in dollars per share)	$ 0.42	$ 0.47
Diluted income per common share:
Net income	$ 178	$ 198
Less: Net income allocable to participating securities	(2)	(2)
Net income allocable to common shares	$ 176	$ 196
Weighted average shares outstanding and common share equivalents:
Weighted average common shares outstanding	415	414
Weighted average effect of share-based payment awards	2	1
Denominator in calculation of diluted income per share	417	415
Diluted income per common share (in dollars per share)	$ 0.42	$ 0.47
Income per common share, Other disclosures [Abstract]
Number of anti-dilutive restricted stock awards and options excluded from the calculation	2	2
Weighted average restricted stock awards considered participating securities	5	5