Installment Receivables (Gross Basis by Credit Category) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Mar. 31, 2015
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	$ 1,387	$ 1,725
Prime [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	1,056	1,329
Subprime [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	331	396
Billed Revenues [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	1,258	1,602
Billed Revenues [Member] | Prime [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	965	1,243
Billed Revenues [Member] | Subprime [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	293	359
Billed receivables, current [Member] | Billed Revenues [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	89	87
Billed receivables, current [Member] | Billed Revenues [Member] | Prime [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	67	65
Billed receivables, current [Member] | Billed Revenues [Member] | Subprime [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	22	22
Billed receivables, past due [Member] | Billed Revenues [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	40	36
Billed receivables, past due [Member] | Billed Revenues [Member] | Prime [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	24	21
Billed receivables, past due [Member] | Billed Revenues [Member] | Subprime [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	$ 16	$ 15