Installment Receivables (Gross Basis by Credit Category) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Mar. 31, 2015
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	$ 1,566	$ 1,725
Prime [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	1,201	1,329
Subprime [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	365	396
Billed Revenues [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	1,455	1,602
Billed Revenues [Member] | Prime [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	1,125	1,243
Billed Revenues [Member] | Subprime [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	330	359
Billed receivables, current [Member] | Billed Revenues [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	72	87
Billed receivables, current [Member] | Billed Revenues [Member] | Prime [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	53	65
Billed receivables, current [Member] | Billed Revenues [Member] | Subprime [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	19	22
Billed receivables, past due [Member] | Billed Revenues [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	39	36
Billed receivables, past due [Member] | Billed Revenues [Member] | Prime [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	23	21
Billed receivables, past due [Member] | Billed Revenues [Member] | Subprime [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	$ 16	$ 15