Installment Receivables - Balance and Aging of Financing Receivables by Credit Category (Details) - USD ($) $ in Millions	Sep. 30, 2018	Mar. 31, 2018
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	$ 1,082	$ 1,472
Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	720	1,037
Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	362	435
Unbilled
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	980	1,342
Unbilled | Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	652	951
Unbilled | Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	328	391
Billed - current | Billed
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	77	98
Billed - current | Billed | Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	55	69
Billed - current | Billed | Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	22	29
Billed - past due | Billed
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	25	32
Billed - past due | Billed | Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	13	17
Billed - past due | Billed | Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	$ 12	$ 15