Installment Receivables - Balance and Aging of Financing Receivables by Credit Category (Details) - USD ($) $ in Millions	Dec. 31, 2019	Mar. 31, 2019
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	$ 1,578	$ 1,212
Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	920	720
Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	658	492
Unbilled
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	1,468	1,126
Unbilled | Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	856	667
Unbilled | Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	612	459
Billed - current | Billed
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	84	65
Billed - current | Billed | Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	53	43
Billed - current | Billed | Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	31	22
Billed - past due | Billed
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	26	21
Billed - past due | Billed | Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	11	10
Billed - past due | Billed | Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	$ 15	$ 11