Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Basic income per common share:
Net income	$ 166	$ 178
Less: Net income allocable to participating securities	(2)	(2)
Net income allocable to common shares	$ 164	$ 176
Weighted average common shares outstanding	413	415
Basic income per common share (in dollars per share)	$ 0.40	$ 0.42
Diluted income per common share:
Net income	$ 166	$ 178
Less: Net income allocable to participating securities	(2)	(2)
Net income allocable to common shares	$ 164	$ 176
Weighted average shares outstanding and common share equivalents:
Weighted average common shares outstanding	413	415
Weighted average effect of share-based payment awards	2	2
Denominator in calculation of diluted income per share	415	417
Diluted income per common share (in dollars per share)	$ 0.40	$ 0.42
Income per common share, Other disclosures
Number of anti-dilutive restricted stock awards and options excluded from the calculation	1	2
Weighted average restricted stock awards considered participating securities	5	5