Summary of Significant Accounting Policies (Advertising Costs) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019
Operating Expenses [Abstract]
Advertising expense, net of vendor reimbursements	$ 222	$ 195	$ 271
Cooperative Advertising Amount	$ 22	$ 38	$ 46