Equipment Installment Plans (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2016	Dec. 31, 2015
Equipment installment plan receivables
Guarantee liability	$ 77	$ 93
Imputed interest rate	9.90%	9.70%
Short-term
Equipment installment plan receivables
Unbilled equipment installment plan receivables, gross	$ 300	$ 279
Unbilled deferred interest	(23)	(21)
Unbilled allowance for credit losses	(16)	(14)
Unbilled equipment installment plan receivables, net	261	244
Long-term
Equipment installment plan receivables
Unbilled equipment installment plan receivables, gross	91	76
Unbilled deferred interest	(3)	(2)
Unbilled allowance for credit losses	(7)	(6)
Unbilled equipment installment plan receivables, net	$ 81	$ 68

Unearned discounts (other than cash or quantity discounts and the like), finance charges, and interest included in the face amount of receivables, that are shown as a deduction from the related receivables. For example, 1) finance charges booked as a receivable when a loan is made and recognized as income at a later date; and 2) interest charges deducted from the face loan amount, resulting in a discounted amount actually advanced to the borrower (wherein the receivable includes the amount actually advanced to the borrower and the as yet unearned interest income).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4519-111522

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A valuation allowance for financing receivables that are expected to be uncollectible.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
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This item represents a non-contingent liability for the fair value of an obligation to stand ready to perform over the term of a guaranty issued in the event that specified triggering events or conditions occur.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 25
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-SubTopic 210
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Amount representing an agreement for an unconditional promise by the maker to pay the entity (holder) a definite sum of money at a future date. Such amount may include accrued interest receivable in accordance with the terms of the note. The note also may contain provisions and related items including a discount or premium, payable on demand, secured, or unsecured, interest bearing or noninterest bearing, among a myriad of other features and characteristics. Excludes amounts related to receivables held-for-sale.

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Net amount of the investment in a contractual right to receive money on demand or on fixed or determinable dates that is recognized as an asset in the creditor's statement of financial position. Examples include, but are not limited to, credit card receivables, notes receivable and receivables relating to lessor's rights to payments from leases other than operating leases that have been recorded as assets. Excludes trade accounts receivable with contractual maturity of one year or less and arose from the sale of goods or services.

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Yield on the receivable, on which interest has been imputed, as calculated from its issuance value or purchase price. The calculated effective interest rate considers factors such as the issued face value or price paid for the receivable, the time period between payments, and the time until maturity [full receipt] of the receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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