Debt (Schedule Of General Information Of Financing Transaction) (Details) (USD $)	9 Months Ended	6 Months Ended	9 Months Ended	15 Months Ended	9 Months Ended										6 Months Ended		9 Months Ended		15 Months Ended	6 Months Ended	9 Months Ended		15 Months Ended	9 Months Ended														9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013 Revolving Secured Line Of Credit [Member]	Sep. 30, 2013 Revolving Secured Line Of Credit [Member]	Mar. 31, 2013 Revolving Secured Line Of Credit [Member]	Sep. 30, 2013 Revolving Secured Line Of Credit [Member] LIBOR [Member]	Sep. 30, 2013 Revolving Secured Line Of Credit [Member] Prime Rate [Member]	Sep. 30, 2013 Warehouse Facility II [Member]		Sep. 30, 2013 Warehouse Facility II [Member] LIBOR [Member]		Sep. 30, 2013 Warehouse Facility III [Member]		Sep. 30, 2013 Warehouse Facility III [Member] LIBOR [Member]		Sep. 30, 2013 Warehouse Facility IV [Member]		Sep. 30, 2013 Warehouse Facility IV [Member]		Mar. 31, 2013 Warehouse Facility IV [Member]	Sep. 30, 2013 Warehouse Facility IV [Member] LIBOR [Member]	Sep. 30, 2013 Warehouse Facility IV [Member] LIBOR [Member]		Mar. 31, 2013 Warehouse Facility IV [Member] LIBOR [Member]	Sep. 30, 2013 Term ABS 2010-1 [Member]		Sep. 30, 2013 Term ABS 2011-1 [Member]		Sep. 30, 2013 Term ABS 2012-1 [Member]		Sep. 30, 2013 Term ABS 2012-2 [Member]		Sep. 30, 2013 Term ABS 2013-1 [Member]		Sep. 30, 2013 Senior Notes [Member]		Sep. 30, 2013 $250.0 Million Senior Notes [Member]	Feb. 01, 2010 $250.0 Million Senior Notes [Member]	Sep. 30, 2013 $100.0 Million Senior Notes [Member]	Mar. 03, 2011 $100.0 Million Senior Notes [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary			n/a				CAC Warehouse Funding Corp. II	[1]			CAC Warehouse Funding III, LLC	[1]					CAC Warehouse Funding LLC IV	[1]						Credit Acceptance Funding LLC 2010-1	[1]	Credit Acceptance Funding LLC 2011-1	[1]	Credit Acceptance Funding LLC 2012-1	[1]	Credit Acceptance Funding LLC 2012-2	[1]	Credit Acceptance Funding LLC 2013-1	[1]	n/a	[2]
Close Date			Jun. 20, 2013
Close date, secured financings							Dec. 27, 2012	[1]			Aug. 16, 2013	[1]					Apr. 05, 2013	[1]						Nov. 04, 2010	[1]	Oct. 06, 2011	[1]	Mar. 29, 2012	[1]	Sep. 20, 2012	[1]	Apr. 25, 2013	[1]			Feb. 01, 2010		Mar. 03, 2011
Maturity date	Sep. 10, 2017	Jun. 23, 2016	Jun. 23, 2016	Jun. 22, 2015
Debt maturity date							Dec. 27, 2015	[1],[3]			Sep. 10, 2015	[1],[4]			Apr. 05, 2016		Apr. 05, 2016	[1],[3]	Feb. 19, 2014					Oct. 15, 2012	[1],[3]	Sep. 16, 2013	[1],[3]	Mar. 17, 2014	[1],[3]	Sep. 15, 2014	[1],[3]	Apr. 15, 2015	[1],[3]	Feb. 01, 2017	[2]
Financing Amount		$ 235,000,000	$ 235,000,000
Debt financing amount							$ 325,000,000	[1]			$ 75,000,000	[1]			$ 75,000,000	[1]	$ 75,000,000	[1]						$ 100,500,000	[1]	$ 200,500,000	[1]	$ 201,300,000	[1]	$ 252,000,000	[1]	$ 140,300,000	[1]	$ 350,000,000	[2]	$ 250,000,000	$ 250,000,000	$ 100,000,000	$ 100,000,000
Basis spread on variable rate					1.875%	0.875%			2.00%	[1],[5]			1.60%	[1],[5]						2.25%	2.25%	[1],[5]	2.75%

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

[2] The close dates associated with the issuance of $250.0 million and $100.0 million of the Senior Notes were on February 1, 2010 and March 3, 2011, respectively.

[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, 2017 will be due.(5)The close dates associated with the issuance of $250.0 million and $100.0 million of the Senior Notes were on February 1, 2010 and March 3, 2011, respectively.

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