Debt (Schedule Of General Information Of Financing Transaction) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended																			12 Months Ended
	Mar. 31, 2012 Revolving Secured Line Of Credit [Member]	Dec. 31, 2012 Revolving Secured Line Of Credit [Member]	Dec. 31, 2012 Warehouse Facility II [Member]		Dec. 31, 2012 Warehouse Facility III [Member]		Dec. 31, 2012 Warehouse Facility IV [Member]		Dec. 31, 2012 Term ABS 2010-1 [Member]		Dec. 31, 2012 Term ABS 2011-1 [Member]		Dec. 31, 2012 Term ABS 2012-1 [Member]		Dec. 31, 2012 Term ABS 2012-2 [Member]		Dec. 31, 2012 Senior Notes [Member]	Dec. 31, 2012 $250.0 Million Senior Notes [Member]		Feb. 01, 2010 $250.0 Million Senior Notes [Member]	Dec. 31, 2012 $100.0 Million Senior Notes [Member]		Mar. 03, 2011 $100.0 Million Senior Notes [Member]	Dec. 31, 2012 Prime Rate [Member] Revolving Secured Line Of Credit [Member]	Dec. 31, 2012 LIBOR [Member] Revolving Secured Line Of Credit [Member]	Dec. 31, 2012 LIBOR [Member] Warehouse Facility II [Member]		Dec. 31, 2012 LIBOR [Member] Warehouse Facility III [Member]		Dec. 31, 2012 LIBOR [Member] Warehouse Facility IV [Member]		Dec. 31, 2012 LIBOR [Member] Revolving Secured Line Of Credit [Member]	Mar. 31, 2012 LIBOR [Member] Revolving Secured Line Of Credit [Member]	Dec. 31, 2012 Prime Rate [Member] Revolving Secured Line Of Credit [Member]	Mar. 31, 2012 Prime Rate [Member] Revolving Secured Line Of Credit [Member]	Dec. 31, 2012 Commercial Paper Rate [Member] Warehouse Facility II [Member]	Sep. 30, 2012 Commercial Paper Rate [Member] Warehouse Facility II [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]	n/a
Close Date		Jun. 15, 2012
Close date, secured financings			Dec. 27, 2012	[1]	Jun. 29, 2012	[1]	Aug. 19, 2011	[1]	Nov. 04, 2010	[1]	Oct. 06, 2011	[1]	Mar. 29, 2012	[1]	Sep. 20, 2012	[1]		Feb. 01, 2010	[2]		Mar. 03, 2011	[2]
Maturity Date	Jun. 22, 2014	Jun. 22, 2015
Debt maturity date			Dec. 27, 2015	[1],[3]	Sep. 10, 2015	[1],[4]	Feb. 19, 2014	[1],[3]	Oct. 15, 2012	[1],[3]	Sep. 16, 2013	[1],[3]	Mar. 17, 2014	[1],[3]	Sep. 15, 2014	[1],[3]	Feb. 01, 2017
Financing Amount	$ 205.0	$ 235.0
Debt financing amount			$ 325.0	[1]	$ 75.0	[1]	$ 75.0	[1]	$ 100.5	[1]	$ 200.5	[1]	$ 201.3	[1]	$ 252.0	[1]	$ 350.0			$ 250.0			$ 100.0
Basis spread on variable rate																								0.875%	1.875%	2.00%	[5],[6]	1.60%	[5]	2.75%	[5]	1.875%	2.25%	0.875%	1.25%	2.00%	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 LIBOR rate is used if funding is not available from the commercial paper market.

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