Debt (Narrative) (Details) (USD $)	0 Months Ended	3 Months Ended		9 Months Ended										0 Months Ended		0 Months Ended	9 Months Ended		9 Months Ended
	Jun. 23, 2014	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 Revolving Secured Line of Credit [Member]	Sep. 30, 2014 Warehouse Facilities [Member] facility	Sep. 30, 2014 Warehouse Facility II [Member]		Sep. 30, 2014 Warehouse Facility IV [Member]		Sep. 30, 2014 Warehouse Facility IV And V [Member]	Sep. 15, 2014 Warehouse Facility IV And V [Member]	Feb. 21, 2014 2017 Senior Notes [Member]	Sep. 30, 2014 2017 Senior Notes [Member]	Jan. 22, 2014 2021 Senior Notes [Member]	Sep. 30, 2014 2021 Senior Notes [Member]	Jan. 22, 2014 2021 Senior Notes [Member]	Sep. 30, 2014 Term ABS 2012-1 [Member]		Sep. 30, 2014 Term ABS 2012-2 [Member]		Sep. 30, 2014 Term ABS 2013-1 [Member]		Sep. 30, 2014 Term ABS 2013-2 [Member]		Sep. 30, 2014 Term ABS 2014-1 [Member]		Sep. 30, 2014 Term ABS 2014-2 [Member]
Debt Instrument [Line Items]
Line of credit facility						$ 235,000,000
Percentage of net book value of loans						0.00%	80.00%
Maximum hedging reserve		1,000,000		1,000,000
Number of warehouse facilities							3
Debt facility financing amount							475,000,000	325,000,000	[1]	75,000,000	[1]	75,000,000	75,000,000				300,000,000	300,000,000	201,300,000.0	[1]	252,000,000	[1]	140,300,000.0	[1]	197,800,000.0	[1]	299,000,000	[1]	349,000,000.0	[1]
Wholly-owned Subsidiary						n/a		CAC Warehouse Funding Corp. II	[1]	CAC Warehouse Funding LLC IV	[1]						n/a		Credit Acceptance Funding LLC 2012-1	[1]	Credit Acceptance Funding LLC 2012-2	[1]	Credit Acceptance Funding LLC 2013-1	[1]	Credit Acceptance Funding LLC 2013-2	[1]	Credit Acceptance Funding LLC 2014-1	[1]	Credit Acceptance Funding LLC 2014-2	[1]
Percentage of collections on contributed loans				6.00%
Monthly servicing fee per financing				6.00%
Debt financing close date																			Mar. 29, 2012		Sep. 20, 2012		Apr. 25, 2013		Oct. 31, 2013		Apr. 16, 2014		Sep. 25, 2014
Close date, secured financings																Jan. 22, 2014
Senior notes stated interest rate															9.125%			6.125%
Repayment of senior notes				350,000,000	0									350,000,000
Redemption price, percentage of principal amount redeemed														104.563%
Loss on extinguishment of debt		0	0	21,800,000	0									(21,800,000)
Repayments of other debt	3,600,000			3,800,000	200,000
Debt covenants		$ 1		$ 1

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