Income from Continuing Operations Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Basic income from continuing operations per common share:
Income from continuing operations	$ 476	$ 775	$ 769
Less: Income from continuing operations allocable to participating securities	(6)	(9)	(8)
Income from continuing operations allocable to common shares	$ 470	$ 766	$ 761
Weighted average common shares outstanding	414	414	426
Basic income from continuing operations per common share (in dollars per share)	$ 1.14	$ 1.85	$ 1.79
Diluted income from continuing operations per common share:
Income from continuing operations	$ 476	$ 775	$ 769
Less: Income from continuing operations allocable to participating securities	(6)	(9)	(8)
Income from continuing operations allocable to common shares	$ 470	$ 766	$ 761
Weighted average shares outstanding and common share equivalents:
Weighted average common shares outstanding	414	414	426
Weighted average effect of share-based payment awards	1	1	1
Denominator in calculation of diluted income per share	415	415	427
Diluted income from continuing operations per common share (in dollars per share)	$ 1.13	$ 1.85	$ 1.78
Income from continuing operations per common share, Other disclosures [Abstract]
Number of anti-dilutive restricted stock awards and options excluded from calculation	2	1	2
Weighted average restricted stock awards considered participating securities	5	5	4