Summary of Significant Accounting Policies (Advertising Costs) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Operating Expenses [Abstract]
Advertising expense, net of vendor reimbursements	$ 271	$ 261	$ 281
Cooperative Advertising Amount	$ 46	$ 45	$ 53