Installment Receivables - Balance and Aging of Financing Receivables by Credit Category (Details) - USD ($) $ in Millions	Dec. 31, 2017	Mar. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	$ 1,773	$ 2,270
Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	1,266	1,595
Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	507	675
Unbilled
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	1,625	2,120
Unbilled | Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	1,170	1,501
Unbilled | Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	455	619
Billed - current | Billed
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	107	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	32	36
Billed - past due | Billed
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	41	40
Billed - past due | Billed | Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	21	20
Billed - past due | Billed | Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	$ 20	$ 20