Debt (Narrative) (Details)					3 Months Ended			6 Months Ended
		Mar. 30, 2015 USD ($)	Feb. 21, 2014 USD ($)	Jan. 22, 2014 USD ($)	Jun. 30, 2015 USD ($)		Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) facility		Jun. 30, 2014 USD ($)	Jun. 11, 2015 USD ($)	May. 19, 2015 USD ($)	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 | facility								3
Debt facility financing amount					575,000,000			$ 575,000,000
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					2.19%			2.19%					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					1.78%			1.78%					1.77%
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					$ 1,800,000			$ 1,800,000
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]
Debt facility financing amount	[1]				$ 252,000,000			$ 252,000,000
Wholly-owned Subsidiary	[1]							Credit Acceptance Funding LLC 2012-2
Debt financing close date								Sep. 20, 2012
Debt maturity date	[1],[3]							Sep. 15, 2014
Senior notes yield to maturity					2.01%			2.01%					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					1.33%			1.33%					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	[1]				$ 300,600,000.0			$ 300,600,000.0
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%
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	[1],[3]							2.00%
LIBOR [Member] | Warehouse Facility IV [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	[1],[3]							2.00%
LIBOR [Member] | Warehouse Facility V [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	[1],[3]							1.60%
Prime Rate [Member] | Revolving Secured Line of Credit [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate								0.875%

[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.