Installment Receivables - Balance and Aging of Financing Receivables by Credit Category (Details) - USD ($) $ in Millions	Jun. 30, 2019	Mar. 31, 2019
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	$ 1,332	$ 1,212
Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	775	720
Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	557	492
Unbilled
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	1,243	1,126
Unbilled | Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	722	667
Unbilled | Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	521	459
Billed - current | Billed
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	67	65
Billed - current | Billed | Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	43	43
Billed - current | Billed | Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	24	22
Billed - past due | Billed
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	22	21
Billed - past due | Billed | Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	10	10
Billed - past due | Billed | Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	$ 12	$ 11