Installment Receivables - Balance and Aging of Financing Receivables by Credit Category (Details) - USD ($) $ in Millions	Jun. 30, 2018	Mar. 31, 2018
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	$ 1,232	$ 1,472
Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	859	1,037
Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	373	435
Unbilled
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	1,119	1,342
Unbilled | Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	782	951
Unbilled | Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	337	391
Billed - current | Billed
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	86	98
Billed - current | Billed | Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	62	69
Billed - current | Billed | Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	24	29
Billed - past due | Billed
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	27	32
Billed - past due | Billed | Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	15	17
Billed - past due | Billed | Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	$ 12	$ 15