Accounting for Share-Based Compensation 2 (Details) $ in Millions	6 Months Ended
	Sep. 30, 2017 USD ($)
Unrecognized share-based compensation costs
Unrecognized share-based compensation costs	$ 168
Weighted average period expected to be recognized (in years)	2 years 2 months 12 days
Stock Option Awards
Unrecognized share-based compensation costs
Unrecognized share-based compensation costs	$ 6
Weighted average period expected to be recognized (in years)	2 years 1 month 6 days
Restricted Stock Units (RSUs)
Unrecognized share-based compensation costs
Unrecognized share-based compensation costs	$ 25
Weighted average period expected to be recognized (in years)	2 years 1 month 6 days
Restricted Stock Awards (RSAs)
Unrecognized share-based compensation costs
Unrecognized share-based compensation costs	$ 92
Weighted average period expected to be recognized (in years)	2 years 1 month 6 days
Performance Share Units
Unrecognized share-based compensation costs
Unrecognized share-based compensation costs	$ 45
Weighted average period expected to be recognized (in years)	2 years 7 months 6 days