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.