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Debt (Narrative) (Details) - USD ($) |  |  |  | 3 Months Ended | 9 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 30, 2015 | Feb. 21, 2014 | Jan. 22, 2014 | Sep. 30, 2015 | Sep. 30, 2014 | Sep. 30, 2015 | Sep. 30, 2014 | Jun. 11, 2015 | May. 19, 2015 | Dec. 31, 2014
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum hedging reserve |  |  |  |  | $ 1,000,000 |  |  | $ 1,000,000 |  |  |  |  |
Percentage of collections on contributed loans |  |  |  |  |  |  |  | 6.00% |  |  |  |  |
Monthly servicing fee per financing |  |  |  |  |  |  |  | 6.00% |  |  |  |  |
Repayment of senior notes |  |  |  |  |  |  |  | $ 0 |  | $ 350,000,000 |  |  |
Proceeds from issuance of senior notes |  |  |  |  |  |  |  | 248,200,000 |  | 300,000,000 |  |  |
Loss on extinguishment of debt |  |  |  |  | 0 |  | $ 0 | 0 |  | $ 21,800,000 |  |  |
Debt covenants |  |  |  |  | 1 |  |  | 1 |  |  |  |  |
Revolving Secured Line of Credit [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Line of credit facility |  |  |  |  | 310,000,000 |  |  | $ 310,000,000 |  |  | $ 235,000,000 |  |
Percentage of net book value of loans |  |  |  |  |  |  |  | 0.00% |  |  |  |  |
Wholly-owned Subsidiary |  |  |  |  |  |  |  | n/a |  |  |  |  |
Warehouse Facilities [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of net book value of loans |  |  |  |  |  |  |  | 80.00% |  |  |  |  |
Number of warehouse facilities |  |  |  |  |  |  |  | 4 |  |  |  |  |
Debt facility financing amount |  |  |  |  | 650,000,000 |  |  | $ 650,000,000 |  |  |  |  |
Warehouse Facility II [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt facility financing amount |  |  |  |  | $ 400,000,000 | [1],[2] |  | $ 400,000,000 | [1],[2] |  |  | $ 325,000,000 |
Wholly-owned Subsidiary | [1] |  |  |  |  |  |  | CAC Warehouse Funding Corp. II |  |  |  |  |
Debt maturity date | [1],[3] |  |  |  |  |  |  | Jul. 18, 2017 |  |  |  |  |
Senior notes yield to maturity |  |  |  |  | 2.20% |  |  | 2.20% |  |  |  |  | 2.16%
Warehouse Facility IV [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt facility financing amount | [1],[2] |  |  |  | $ 75,000,000 |  |  | $ 75,000,000 |  |  |  |  |
Wholly-owned Subsidiary | [1] |  |  |  |  |  |  | CAC Warehouse Funding LLC IV |  |  |  |  |
Debt maturity date | [1],[3] |  |  |  |  |  |  | Apr. 30, 2018 |  |  |  |  |
Senior notes yield to maturity |  |  |  |  | 2.19% |  |  | 2.19% |  |  |  |  | 2.17%
Warehouse Facility V [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt facility financing amount |  |  |  |  | $ 100,000,000 | [1],[2] |  | $ 100,000,000 | [1],[2] |  | $ 75,000,000 |  |
Wholly-owned Subsidiary | [1] |  |  |  |  |  |  | CAC Warehouse Funding LLC V |  |  |  |  |
Debt maturity date | [1],[4] |  |  |  |  |  |  | Sep. 10, 2017 |  |  |  |  |
Senior notes yield to maturity |  |  |  |  | 1.80% |  |  | 1.80% |  |  |  |  | 1.77%
Warehouse Facility VI [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt facility financing amount | [1] |  |  |  | $ 75,000,000 |  |  | $ 75,000,000 |  |  |  |  |
Wholly-owned Subsidiary | [1] |  |  |  |  |  |  | CAC Warehouse Funding LLC VI |  |  |  |  |
Debt maturity date | [1],[3] |  |  |  |  |  |  | Sep. 30, 2018 |  |  |  |  |
Senior notes yield to maturity |  |  |  |  | 2.20% |  |  | 2.20% |  |  |  |  | 0.00%
2017 Senior Notes [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Senior notes stated interest rate |  |  |  |  | 9.125% |  |  | 9.125% |  |  |  |  |
Repayment of senior notes |  |  | $ 350,000,000 |  |  |  |  |  |  |  |  |  |
Loss on extinguishment of debt |  |  | $ (21,800,000) |  |  |  |  |  |  |  |  |  |
2021 Senior Notes [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt facility financing amount |  |  |  | $ 300,000,000 | $ 300,000,000 |  |  | $ 300,000,000 |  |  |  |  |
Wholly-owned Subsidiary |  |  |  |  |  |  |  | n/a |  |  |  |  |
Close date, secured financings |  |  |  | Jan. 22, 2014 |  |  |  |  |  |  |  |  |
Senior notes stated interest rate |  |  |  | 6.125% |  |  |  |  |  |  |  |  |
Debt maturity date |  |  |  | Feb. 15, 2021 |  |  |  | Feb. 15, 2021 |  |  |  |  |
Senior notes yield to maturity |  |  |  |  | 6.125% |  |  | 6.125% |  |  |  |  | 6.125%
2023 Senior Notes [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt facility financing amount |  | $ 250,000,000.0 |  |  | $ 250,000,000 |  |  | $ 250,000,000 |  |  |  |  |
Wholly-owned Subsidiary |  |  |  |  |  |  |  | n/a |  |  |  |  |
Close date, secured financings |  | Mar. 30, 2015 |  |  |  |  |  |  |  |  |  |  |
Senior notes stated interest rate |  | 7.375% |  |  |  |  |  |  |  |  |  |  |
Debt maturity date |  | Mar. 15, 2023 |  |  |  |  |  | Mar. 15, 2023 |  |  |  |  |
Redemption price, percentage of principal amount redeemed |  | 99.266% |  |  |  |  |  |  |  |  |  |  |
Proceeds from issuance of senior notes |  | $ 248,200,000 |  |  |  |  |  |  |  |  |  |  |
Senior notes yield to maturity |  | 7.50% |  |  | 7.375% |  |  | 7.375% |  |  |  |  | 0.00%
Unamortized debt discount |  |  |  |  | $ 1,700,000 |  |  | $ 1,700,000 |  |  |  |  |
Term ABS 2012-1 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Senior notes yield to maturity |  |  |  |  | 0.00% |  |  | 0.00% |  |  |  |  | 3.04%
Term ABS 2012-2 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Senior notes yield to maturity |  |  |  |  | 0.00% |  |  | 0.00% |  |  |  |  | 1.67%
Term ABS 2013-1 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt facility financing amount | [1] |  |  |  | $ 140,300,000.0 |  |  | $ 140,300,000.0 |  |  |  |  |
Wholly-owned Subsidiary | [1] |  |  |  |  |  |  | Credit Acceptance Funding LLC 2013-1 |  |  |  |  |
Debt financing close date |  |  |  |  |  |  |  | Apr. 25, 2013 |  |  |  |  |
Debt maturity date | [1],[3] |  |  |  |  |  |  | Apr. 15, 2015 |  |  |  |  |
Senior notes yield to maturity |  |  |  |  | 1.40% |  |  | 1.40% |  |  |  |  | 1.31%
Term ABS 2013-2 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt facility financing amount | [1] |  |  |  | $ 197,800,000.0 |  |  | $ 197,800,000.0 |  |  |  |  |
Wholly-owned Subsidiary | [1] |  |  |  |  |  |  | Credit Acceptance Funding LLC 2013-2 |  |  |  |  |
Debt financing close date |  |  |  |  |  |  |  | Oct. 31, 2013 |  |  |  |  |
Debt maturity date | [1],[3] |  |  |  |  |  |  | Oct. 15, 2015 |  |  |  |  |
Senior notes yield to maturity |  |  |  |  | 1.67% |  |  | 1.67% |  |  |  |  | 1.67%
Term ABS 2014-1 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt facility financing amount | [1] |  |  |  | $ 299,000,000 |  |  | $ 299,000,000 |  |  |  |  |
Wholly-owned Subsidiary | [1] |  |  |  |  |  |  | Credit Acceptance Funding LLC 2014-1 |  |  |  |  |
Debt financing close date |  |  |  |  |  |  |  | Apr. 16, 2014 |  |  |  |  |
Debt maturity date | [1],[3] |  |  |  |  |  |  | Apr. 15, 2016 |  |  |  |  |
Senior notes yield to maturity |  |  |  |  | 1.72% |  |  | 1.72% |  |  |  |  | 1.72%
Term ABS 2014-2 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt facility financing amount | [1] |  |  |  | $ 349,000,000.0 |  |  | $ 349,000,000.0 |  |  |  |  |
Wholly-owned Subsidiary | [1] |  |  |  |  |  |  | Credit Acceptance Funding LLC 2014-2 |  |  |  |  |
Debt financing close date |  |  |  |  |  |  |  | Sep. 25, 2014 |  |  |  |  |
Debt maturity date | [1],[3] |  |  |  |  |  |  | Sep. 15, 2016 |  |  |  |  |
Senior notes yield to maturity |  |  |  |  | 2.05% |  |  | 2.05% |  |  |  |  | 2.05%
Term ABS 2015-1 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt facility financing amount |  |  |  |  | $ 300,600,000 |  |  | $ 300,600,000 |  |  |  |  |
Wholly-owned Subsidiary | [1] |  |  |  |  |  |  | Credit Acceptance Funding LLC 2015-1 |  |  |  |  |
Debt financing close date |  |  |  |  |  |  |  | Jan. 29, 2015 |  |  |  |  |
Debt maturity date | [1],[3] |  |  |  |  |  |  | Jan. 16, 2017 |  |  |  |  |
Senior notes yield to maturity |  |  |  |  | 2.26% |  |  | 2.26% |  |  |  |  |
Term ABS 2015-2 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt facility financing amount |  |  |  |  | $ 300,200,000 |  |  | $ 300,200,000 |  |  |  |  |
Wholly-owned Subsidiary | [1] |  |  |  |  |  |  | Credit Acceptance Funding LLC 2015-2 |  |  |  |  |
Debt financing close date |  |  |  |  |  |  |  | Aug. 20, 2015 |  |  |  |  |
Debt maturity date | [1],[3] |  |  |  |  |  |  | Aug. 15, 2017 |  |  |  |  |
Senior notes yield to maturity |  |  |  |  | 2.63% |  |  | 2.63% |  |  |  |  |
[1] | Financing made available only to a specified subsidiary of the Company.
[2] | Interest rate cap agreements are in place to limit the exposure to increasing interest rates.
[3] | Represents the revolving maturity date. The outstanding balance will amortize after the maturity date based on the cash flows of the pledged assets.
[4] | Represents the revolving maturity date. The outstanding balance will amortize after the revolving maturity date and any amounts remaining on September 10, 2019 will be due.

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