Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Basic income per common share:
Net income	$ 129	$ 184	$ 295	$ 362
Less: Net income allocable to participating securities	(2)	(3)	(4)	(5)
Net income allocable to common shares	$ 127	$ 181	$ 291	$ 357
Weighted average common shares outstanding	413	415	414	415
Basic income per common share (in dollars per share)	$ 0.31	$ 0.44	$ 0.70	$ 0.86
Diluted income per common share:
Net income	$ 129	$ 184	$ 295	$ 362
Less: Net income allocable to participating securities	(2)	(3)	(4)	(5)
Net income allocable to common shares	$ 127	$ 181	$ 291	$ 357
Weighted average shares outstanding and common share equivalents:
Weighted average common shares outstanding	413	415	414	415
Weighted average effect of share-based payment awards	3	1	2	1
Denominator in calculation of diluted income per share	416	416	416	416
Diluted income per common share (in dollars per share)	$ 0.31	$ 0.44	$ 0.70	$ 0.86
Income per common share, Other disclosures
Number of anti-dilutive restricted stock awards and options excluded from the calculation	0	2	1	2
Weighted average restricted stock awards considered participating securities	5	5	5	5