Installment Receivables (Schedule of Balance and Aging of Installment Receivables) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Installment receivables, gross	$ 1,212	$ 1,472
Unbilled Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Installment receivables, gross	1,126	1,342
Subprime [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Installment receivables, gross	492	435
Subprime [Member] | Unbilled Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Installment receivables, gross	459	391
Prime [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Installment receivables, gross	720	1,037
Prime [Member] | Unbilled Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Installment receivables, gross	667	951
Billed receivables, current [Member] | Billed Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Installment receivables, gross	65	98
Billed receivables, current [Member] | Subprime [Member] | Billed Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Installment receivables, gross	22	29
Billed receivables, current [Member] | Prime [Member] | Billed Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Installment receivables, gross	43	69
Billed receivables, past due [Member] | Billed Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Installment receivables, gross	21	32
Billed receivables, past due [Member] | Subprime [Member] | Billed Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Installment receivables, gross	11	15
Billed receivables, past due [Member] | Prime [Member] | Billed Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Installment receivables, gross	10	17
Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Write-offs, net of recoveries	$ (118)	$ (224)