Installment Receivables (Gross Basis by Credit Category) (Details) $ in Millions	Mar. 31, 2015 USD ($)
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	$ 1,725
Prime [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	1,329
Subprime [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	396
Billed Revenues [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	1,602
Billed Revenues [Member] | Prime [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	1,243
Billed Revenues [Member] | Subprime [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	359
Billed receivables, current [Member] | Billed Revenues [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	87
Billed receivables, current [Member] | Billed Revenues [Member] | Prime [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	65
Billed receivables, current [Member] | Billed Revenues [Member] | Subprime [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	22
Billed receivables, past due [Member] | Billed Revenues [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	36
Billed receivables, past due [Member] | Billed Revenues [Member] | Prime [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	21
Billed receivables, past due [Member] | Billed Revenues [Member] | Subprime [Member]
Financing Receivable, Recorded Investment [Line Items]
Installment receivables, gross	$ 15