Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Basic (loss) income per common share:
Net (loss) income	$ (93)	$ 208	$ 269	$ 618
Less: Net income allocable to participating securities	0	(2)	(5)	(7)
Net (loss) income allocable to common shares	$ (93)	$ 206	$ 264	$ 611
Weighted average common shares outstanding	413	413	414	414
Basic (loss) income per common share (in dollars per share)	$ (0.23)	$ 0.50	$ 0.64	$ 1.48
Diluted (loss) income per common share:
Net (loss) income	$ (93)	$ 208	$ 269	$ 618
Less: Net income allocable to participating securities	0	(2)	(5)	(7)
Net (loss) income allocable to common shares	$ (93)	$ 206	$ 264	$ 611
Weighted average shares outstanding and common share equivalents:
Weighted average common shares outstanding	413	413	414	414
Weighted average effect of share-based payment awards	0	1	1	1
Denominator in calculation of diluted income per share	413	414	415	415
Diluted (loss) income per common share (in dollars per share)	$ (0.23)	$ 0.50	$ 0.64	$ 1.47
Income per common share, Other disclosures [Abstract]
Number of anti-dilutive restricted stock awards and options excluded from the calculation	2	1	2	2
Weighted average restricted stock awards considered participating securities	5	5	5	5