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Debt (Narrative) (Details) - USD ($) |  |  |  |  | 9 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Aug. 06, 2018 | Mar. 30, 2015 | Feb. 21, 2014 | Jan. 22, 2014 | Sep. 30, 2018 | Dec. 31, 2017
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Maximum hedging reserve |  |  |  |  |  | $ 1,000,000 |  |
Debt facility financing amount |  |  |  |  |  | 3,901,700,000 |  | $ 3,093,000,000
Deferred Debt Issuance Costs |  |  |  |  |  | $ (23,600,000) |  | (20,900,000)
Percentage of collections on contributed loans |  |  |  |  |  | 6.00% |  |
Monthly servicing fee per financing |  |  |  |  |  | 6.00% |  |
Mortgage note |  |  |  |  |  | $ 12,000,000 | [1] | 0
Debt covenants |  |  |  |  |  | 1 |  |
Unamortized debt discount |  |  |  |  |  | (1,200,000) |  | (1,300,000)
Carrying Amount |  |  |  |  |  | 3,876,900,000 |  | 3,070,800,000
Senior Notes [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt facility financing amount |  |  |  |  |  | 550,000,000 |  | 550,000,000
Deferred Debt Issuance Costs |  |  |  |  |  | (4,800,000) |  | (5,900,000)
Unamortized debt discount |  |  |  |  |  | (1,200,000) |  | (1,300,000)
Carrying Amount |  |  |  |  |  | 544,000,000 |  | $ 542,800,000
Mortgages [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt facility financing amount |  | $ 12,000,000.0 |  |  |  | 12,000,000 |  |
Deferred Debt Issuance Costs |  |  |  |  |  | $ 0 |  |
Wholly-owned Subsidiary |  |  |  |  |  | Chapter 4 Properties, LLC |  |
Close date, secured financings |  | Aug. 06, 2018 |  |  |  |  |  |
Debt maturity date |  |  |  |  |  | Aug. 06, 2023 |  |
Senior notes yield to maturity |  |  |  |  |  | 3.61% |  | 0.00%
Unamortized debt discount |  |  |  |  |  | $ 0 |  |
Carrying Amount |  |  |  |  |  | 12,000,000 |  |
Revolving Secured Line of Credit [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Line of credit facility | [2] |  |  |  |  | $ 350,000,000 |  |
Percentage of net book value of loans |  |  |  |  |  | 0.00% |  |
Debt facility financing amount | [3] |  |  |  |  | $ 0 |  | $ 13,900,000
Deferred Debt Issuance Costs | [3] |  |  |  |  | $ 0 |  | 0
Wholly-owned Subsidiary |  |  |  |  |  | n/a |  |
Unamortized debt discount | [3] |  |  |  |  | $ 0 |  | 0
Carrying Amount | [3] |  |  |  |  | 0 |  | $ 13,900,000
Line of Credit Two Thousand Nineteen [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Line of credit facility |  |  |  |  |  | $ 315,000,000 |  |
Warehouse Facilities [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Percentage of net book value of loans |  |  |  |  |  | 80.00% |  |
Number of warehouse facilities |  |  |  |  |  | 5 |  |
Debt facility financing amount |  |  |  |  |  | $ 975,000,000 |  |
Warehouse Facility II [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt facility financing amount | [4],[5] |  |  |  |  | $ 400,000,000 |  |
Wholly-owned Subsidiary | [4] |  |  |  |  | CAC Warehouse Funding Corp. II |  |
Debt maturity date | [4],[5] |  |  |  |  | Dec. 20, 2020 |  |
Senior notes yield to maturity |  |  |  |  |  | 0.00% |  | 0.00%
Warehouse Facility IV [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt facility financing amount |  |  |  |  |  | $ 250,000,000 | [4],[5] | $ 100,000,000
Wholly-owned Subsidiary | [4] |  |  |  |  | CAC Warehouse Funding LLC IV |  |
Debt maturity date | [4],[5] |  |  |  |  | Apr. 30, 2020 |  |
Senior notes yield to maturity |  |  |  |  |  | 0.00% |  | 0.00%
Warehouse Facility V [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt facility financing amount | [4],[5] |  |  |  |  | $ 100,000,000 |  |
Wholly-owned Subsidiary | [4] |  |  |  |  | CAC Warehouse Funding LLC V |  |
Debt maturity date | [4],[6] |  |  |  |  | Aug. 17, 2021 |  |
Senior notes yield to maturity |  |  |  |  |  | 0.00% |  | 0.00%
Warehouse Facility VI [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt facility financing amount | [4] |  |  |  |  | $ 75,000,000 |  |
Wholly-owned Subsidiary | [4] |  |  |  |  | CAC Warehouse Funding LLC VI |  |
Debt maturity date | [4],[5] |  |  |  |  | Sep. 30, 2020 |  |
Senior notes yield to maturity |  |  |  |  |  | 0.00% |  | 0.00%
Warehouse Facility VII [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt facility financing amount | [4] |  |  |  |  | $ 150,000,000 |  |
Wholly-owned Subsidiary | [4] |  |  |  |  | CAC Warehouse Funding LLC VII |  |
Debt maturity date | [4],[7] |  |  |  |  | Dec. 01, 2019 |  |
Senior notes yield to maturity |  |  |  |  |  | 0.00% |  | 0.00%
2017 Senior Notes [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Senior notes stated interest rate |  |  |  | 9.125% |  |  |  |
Repayment of senior notes |  |  |  | $ 350,000,000 |  |  |  |
2021 Senior Notes [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt facility financing amount |  |  |  |  | $ 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%
2023 Senior Notes [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt facility financing amount |  |  | $ 250,000,000.0 |  |  | $ 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%
Term ABS 2015-1 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Senior notes yield to maturity |  |  |  |  |  | 0.00% |  | 2.88%
Term ABS 2015-2 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt facility financing amount | [4] |  |  |  |  | $ 300,200,000 |  |
Wholly-owned Subsidiary | [4] |  |  |  |  | Credit Acceptance Funding LLC 2015-2 |  |
Debt financing close date |  |  |  |  |  | Aug. 20, 2015 |  |
Debt maturity date | [4],[5] |  |  |  |  | Aug. 15, 2017 |  |
Senior notes yield to maturity |  |  |  |  |  | 3.31% |  | 2.72%
Net Book Value Of Loans Pledged As Collateral |  |  |  |  |  | $ 375,500,000 |  |
Term ABS 2016-1 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt facility financing amount | [4],[5] |  |  |  |  | $ 385,000,000 |  |
Wholly-owned Subsidiary | [4] |  |  |  |  | Credit Acceptance Funding LLC 2016-1 |  |
Debt financing close date |  |  |  |  |  | Feb. 26, 2016 |  |
Debt maturity date | [4],[5] |  |  |  |  | Feb. 15, 2018 |  |
Senior notes yield to maturity |  |  |  |  |  | 4.11% |  | 3.18%
Net Book Value Of Loans Pledged As Collateral |  |  |  |  |  | $ 481,400,000 |  |
Term ABS 2016-2 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt facility financing amount | [4] |  |  |  |  | $ 350,200,000 |  |
Wholly-owned Subsidiary | [4] |  |  |  |  | Credit Acceptance Funding LLC 2016-2 |  |
Debt financing close date |  |  |  |  |  | May 12, 2016 |  |
Debt maturity date | [4],[5] |  |  |  |  | May 15, 2018 |  |
Senior notes yield to maturity |  |  |  |  |  | 2.99% |  | 2.83%
Net Book Value Of Loans Pledged As Collateral |  |  |  |  |  | $ 437,800,000 |  |
Term ABS 2016-3 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt facility financing amount | [4] |  |  |  |  | $ 350,000,000 |  |
Wholly-owned Subsidiary | [4] |  |  |  |  | Credit Acceptance Funding LLC 2016-3 |  |
Debt financing close date |  |  |  |  |  | Oct. 27, 2016 |  |
Debt maturity date | [4],[5] |  |  |  |  | Oct. 15, 2018 |  |
Senior notes yield to maturity |  |  |  |  |  | 2.53% |  | 2.53%
Net Book Value Of Loans Pledged As Collateral |  |  |  |  |  | $ 437,800,000 |  |
Term ABS 2017-1 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt facility financing amount | [4] |  |  |  |  | $ 350,000,000 |  |
Wholly-owned Subsidiary | [4] |  |  |  |  | Credit Acceptance Funding LLC 2017-1 |  |
Debt financing close date |  |  |  |  |  | Feb. 23, 2017 |  |
Debt maturity date | [4],[5] |  |  |  |  | Feb. 15, 2019 |  |
Senior notes yield to maturity |  |  |  |  |  | 2.78% |  | 2.78%
Net Book Value Of Loans Pledged As Collateral |  |  |  |  |  | $ 437,800,000 |  |
Term ABS 2017-2 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt facility financing amount | [4] |  |  |  |  | $ 450,000,000 |  |
Wholly-owned Subsidiary | [4] |  |  |  |  | Credit Acceptance Funding LLC 2017-2 |  |
Debt financing close date |  |  |  |  |  | Jun. 29, 2017 |  |
Debt maturity date | [4],[5] |  |  |  |  | Jun. 17, 2019 |  |
Senior notes yield to maturity |  |  |  |  |  | 2.72% |  | 2.72%
Net Book Value Of Loans Pledged As Collateral |  |  |  |  |  | $ 563,200,000 |  |
Term ABS 2017-3 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt facility financing amount | [4] |  |  |  |  | $ 350,000,000 |  |
Wholly-owned Subsidiary | [4] |  |  |  |  | Credit Acceptance Funding LLC 2017-3 |  |
Debt financing close date |  |  |  |  |  | Oct. 26, 2017 |  |
Debt maturity date | [4],[5] |  |  |  |  | Oct. 15, 2019 |  |
Senior notes yield to maturity |  |  |  |  |  | 2.88% |  | 2.88%
Net Book Value Of Loans Pledged As Collateral |  |  |  |  |  | $ 437,600,000 |  |
Term ABS 2018-1 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt facility financing amount | [4] |  |  |  |  | $ 500,000,000 |  |
Wholly-owned Subsidiary | [4] |  |  |  |  | Credit Acceptance Funding LLC 2018-1 |  |
Debt financing close date |  |  |  |  |  | Feb. 22, 2018 |  |
Debt maturity date | [4],[5] |  |  |  |  | Feb. 17, 2020 |  |
Senior notes yield to maturity |  |  |  |  |  | 3.24% |  |
Net Book Value Of Loans Pledged As Collateral |  |  |  |  |  | $ 625,100,000 |  |
Term ABS 2018-2 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt facility financing amount | [4] |  |  |  |  | $ 450,000,000 |  |
Wholly-owned Subsidiary | [4] |  |  |  |  | Credit Acceptance Funding LLC 2018-2 |  |
Debt financing close date |  |  |  |  |  | May 24, 2018 |  |
Debt maturity date | [4],[5] |  |  |  |  | May 15, 2020 |  |
Senior notes yield to maturity |  |  |  |  |  | 3.68% |  |
Net Book Value Of Loans Pledged As Collateral |  |  |  |  |  | $ 562,600,000 |  |
Term ABS 2018-3 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt facility financing amount | [4] |  |  |  |  | $ 398,300,000 |  |
Wholly-owned Subsidiary | [4] |  |  |  |  | Credit Acceptance Funding LLC 2018-3 |  |
Debt financing close date |  |  |  |  |  | Aug. 23, 2018 |  |
Debt maturity date | [4],[5] |  |  |  |  | Aug. 17, 2020 |  |
Senior notes yield to maturity |  |  |  |  |  | 3.72% |  |
Net Book Value Of Loans Pledged As Collateral |  |  |  |  |  | $ 500,100,000 |  |
LIBOR [Member] | Mortgages [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate | [4],[7] |  |  |  |  | 1.50% |  |
LIBOR [Member] | Revolving Secured Line of Credit [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  |  |  |  |  | 1.875% |  |
LIBOR [Member] | Warehouse Facility II [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate | [4],[7] |  |  |  |  | 1.75% |  |
LIBOR [Member] | Warehouse Facility IV [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate | [4],[7] |  |  |  |  | 2.25% |  |
LIBOR [Member] | Warehouse Facility V [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate | [4],[7] |  |  |  |  | 1.90% |  |
LIBOR [Member] | Warehouse Facility VI [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate | [4] |  |  |  |  | 2.00% |  |
LIBOR [Member] | Term ABS 2016-1 [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate | [4],[7] |  |  |  |  | 1.95% |  |
Prime Rate [Member] | Revolving Secured Line of Credit [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  |  |  |  |  | 0.875% |  |
[1] | {F|ahBzfndlYmZpbGluZ3MtaHJkcmoLEgZYTUxEb2MiXlhCUkxEb2NHZW5JbmZvOmZjOGQ1MTk3NTBkNzRkNDQ4OGM0NDEyMmU4YzVmYzEyfFRleHRTZWxlY3Rpb246Q0Y4QzM0RjE2RkVCQjdCMUNBNDExOUVBNThDQTAwRjMM}
[2] | The amount of the facility will decrease to $315.0 million on June 22, 2019.
[3] | Warehouse facilities and Term ABS.
[4] | Financing made available only to a specified subsidiary of the Company.
[5] | Represents the revolving maturity date. The outstanding balance will amortize after the revolving maturity date based on the cash flows of the pledged assets.
[6] | Represents the revolving maturity date. The outstanding balance will amortize after the revolving maturity date and any amounts remaining on August 17, 2023 will be due on that date.
[7] | Interest rate cap agreements are in place to limit the exposure to increasing interest rates.

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