Installment Receivables - Balance and Aging of Financing Receivables by Credit Category (Details) - USD ($) $ in Millions	Jun. 30, 2017	Mar. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	$ 2,302	$ 2,270
Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	1,641	1,595
Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	661	675
Unbilled
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	2,136	2,120
Unbilled | Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	1,536	1,501
Unbilled | Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	600	619
Billed - current | Billed
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	121	110
Billed - current | Billed | Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	83	74
Billed - current | Billed | Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	38	36
Billed - past due | Billed
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	45	40
Billed - past due | Billed | Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	22	20
Billed - past due | Billed | Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	$ 23	$ 20