Installment Receivables - Balance and Aging of Financing Receivables by Credit Category (Details) - USD ($) $ in Millions	Sep. 30, 2017	Mar. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	$ 2,017	$ 2,270
Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	1,452	1,595
Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	565	675
Unbilled
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	1,861	2,120
Unbilled | Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	1,354	1,501
Unbilled | Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	507	619
Billed - current | Billed
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	110	110
Billed - current | Billed | Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	75	74
Billed - current | Billed | Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	35	36
Billed - past due | Billed
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	46	40
Billed - past due | Billed | Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	23	20
Billed - past due | Billed | Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	$ 23	$ 20