Installment Receivables - Balance and Aging of Financing Receivables by Credit Category (Details) - USD ($) $ in Millions	Dec. 31, 2018	Mar. 31, 2018
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	$ 1,151	$ 1,472
Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	721	1,037
Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	430	435
Unbilled
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	1,057	1,342
Unbilled | Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	661	951
Unbilled | Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	396	391
Billed - current | Billed
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	71	98
Billed - current | Billed | Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	48	69
Billed - current | Billed | Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	23	29
Billed - past due | Billed
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	23	32
Billed - past due | Billed | Prime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	12	17
Billed - past due | Billed | Subprime
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	$ 11	$ 15