Debt (Narrative) (Details) - USD ($)					9 Months Ended
		Mar. 30, 2015	Feb. 21, 2014	Jan. 22, 2014	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Maximum hedging reserve					$ 1,000,000
Debt facility financing amount					$ 3,020,200,000	$ 2,622,100,000
Percentage of collections on contributed loans					6.00%
Monthly servicing fee per financing					6.00%
Debt covenants					$ 1
Unamortized debt discount					1,400,000	1,600,000
Revolving Secured Line of Credit [Member]
Debt Instrument [Line Items]
Line of credit facility	[1]				$ 345,000,000
Percentage of net book value of loans					0.00%
Debt facility financing amount	[2]				$ 130,500,000	0
Wholly-owned Subsidiary					n/a
Unamortized debt discount	[2]				$ 0	$ 0
Revolving Secured Line of Credit 2019 [Member]
Debt Instrument [Line Items]
Line of credit facility					$ 300,000,000
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					$ 675,000,000
Warehouse Facility II [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[3],[4]				$ 400,000,000
Wholly-owned Subsidiary	[3]				CAC Warehouse Funding Corp. II
Debt maturity date	[3],[5]				Jun. 23, 2019
Senior notes yield to maturity					0.00%	0.00%
Warehouse Facility IV [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[3],[4]				$ 100,000,000
Wholly-owned Subsidiary	[3]				CAC Warehouse Funding LLC IV
Debt maturity date	[3],[5]				Apr. 30, 2020
Senior notes yield to maturity					0.00%	2.77%
Warehouse Facility V [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[3],[4]				$ 100,000,000
Wholly-owned Subsidiary	[3]				CAC Warehouse Funding LLC V
Debt maturity date	[3],[6]				Aug. 18, 2019
Senior notes yield to maturity					0.00%	0.00%
Warehouse Facility VI [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[3]				$ 75,000,000
Wholly-owned Subsidiary	[3]				CAC Warehouse Funding LLC VI
Debt maturity date	[3],[5]				Sep. 30, 2020
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 2014-2 [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[3]				$ 349,000,000.0
Wholly-owned Subsidiary	[3]				Credit Acceptance Funding LLC 2014-2
Debt financing close date					Sep. 25, 2014
Debt maturity date	[3],[5]				Sep. 15, 2016
Senior notes yield to maturity					2.67%	2.10%
Net Book Value Of Loans Pledged As Collateral					$ 437,600,000
Term ABS 2015-1 [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[3]				$ 300,600,000
Wholly-owned Subsidiary	[3]				Credit Acceptance Funding LLC 2015-1
Debt financing close date					Jan. 29, 2015
Debt maturity date	[3],[5]				Jan. 16, 2017
Senior notes yield to maturity					2.60%	2.26%
Net Book Value Of Loans Pledged As Collateral					$ 375,900,000
Term ABS 2015-2 [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[3]				$ 300,200,000
Wholly-owned Subsidiary	[3]				Credit Acceptance Funding LLC 2015-2
Debt financing close date					Aug. 20, 2015
Debt maturity date	[3],[5]				Aug. 15, 2017
Senior notes yield to maturity					2.65%	2.63%
Net Book Value Of Loans Pledged As Collateral					$ 375,500,000
Term ABS 2016-1 [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[3],[4]				$ 385,000,000
Wholly-owned Subsidiary	[3]				Credit Acceptance Funding LLC 2016-1
Debt financing close date					Feb. 26, 2016
Debt maturity date	[3],[5]				Feb. 15, 2018
Senior notes yield to maturity					3.18%	2.65%
Net Book Value Of Loans Pledged As Collateral					$ 481,400,000
Term ABS 2016-2 [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[3]				$ 350,200,000
Wholly-owned Subsidiary	[3]				Credit Acceptance Funding LLC 2016-2
Debt financing close date					May 12, 2016
Debt maturity date	[3],[5]				May 15, 2018
Senior notes yield to maturity					2.83%	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	[3]				$ 350,000,000
Wholly-owned Subsidiary	[3]				Credit Acceptance Funding LLC 2016-3
Debt financing close date					Oct. 27, 2016
Debt maturity date	[3],[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	[3]				$ 350,000,000
Wholly-owned Subsidiary	[3]				Credit Acceptance Funding LLC 2017-1
Debt financing close date					Feb. 23, 2017
Debt maturity date	[3],[5]				Feb. 15, 2019
Senior notes yield to maturity					2.78%	0.00%
Net Book Value Of Loans Pledged As Collateral					$ 437,800,000
Term ABS 2017-2 [Member]
Debt Instrument [Line Items]
Debt facility financing amount	[3]				$ 450,000,000
Wholly-owned Subsidiary	[3]				Credit Acceptance Funding LLC 2017-2
Debt financing close date					Jun. 29, 2017
Debt maturity date	[3],[5]				Jun. 17, 2019
Senior notes yield to maturity					2.72%	0.00%
Net Book Value Of Loans Pledged As Collateral					$ 563,200,000

[1] Under the terms of the revolving secured line of credit facility as of September 30, 2017, the amount of the facility would decrease to $300.0 million on June 22, 2019.

[2] Warehouse facilities and Term ABS.

[3] Financing made available only to a specified subsidiary of the Company.

[4] Interest rate cap agreements are in place to limit the exposure to increasing interest rates.

[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 18, 2021 will be due on that date.