Installment Receivables (Schedule of Balance and Aging of Installment Receivables) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Installment receivables, gross	$ 1,472	$ 2,270
Unbilled Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Installment receivables, gross	1,342	2,120
Subprime [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Installment receivables, gross	435	675
Subprime [Member] | Unbilled Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Installment receivables, gross	391	619
Prime [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Installment receivables, gross	1,037	1,595
Prime [Member] | Unbilled Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Installment receivables, gross	951	1,501
Billed receivables, current [Member] | Billed Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Installment receivables, gross	98	110
Billed receivables, current [Member] | Subprime [Member] | Billed Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Installment receivables, gross	29	36
Billed receivables, current [Member] | Prime [Member] | Billed Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Installment receivables, gross	69	74
Billed receivables, past due [Member] | Billed Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Installment receivables, gross	32	40
Billed receivables, past due [Member] | Subprime [Member] | Billed Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Installment receivables, gross	15	20
Billed receivables, past due [Member] | Prime [Member] | Billed Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Installment receivables, gross	$ 17	$ 20