Debt (Schedule Of General Information Of Financing Transaction) (Details) - USD ($)				12 Months Ended
		Mar. 30, 2015	Jan. 22, 2014	Dec. 31, 2017		Dec. 31, 2016
Debt Instrument [Line Items]
Debt financing amount				$ 3,093,000,000		$ 2,622,100,000
Revolving Secured Line Of Credit [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary				n/a
Debt maturity date				Jun. 22, 2020
Financing Amount	[1]			$ 350,000,000
Debt financing amount	[2]			$ 13,900,000		0
Revolving Secured Line Of Credit [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate				1.875%
Revolving Secured Line Of Credit [Member] | Prime Rate [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate				0.875%
Warehouse Facility II [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[3],[4]			CAC Warehouse Funding Corp. II
Debt maturity date	[3],[5]			Dec. 20, 2020
Debt financing amount				$ 400,000,000	[3],[4]	400,000,000
Warehouse Facility II [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate				1.75%
Warehouse Facility IV [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[3],[4]			CAC Warehouse Funding LLC IV
Debt maturity date	[3],[5]			Apr. 30, 2020
Debt financing amount				$ 100,000,000	[3],[4]	75,000,000
Warehouse Facility IV [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate				2.25%
Warehouse Facility V [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[3],[4]			CAC Warehouse Funding LLC V
Debt maturity date	[3],[6]			Aug. 18, 2019
Debt financing amount				$ 100,000,000	[3],[4]	100,000,000
Warehouse Facility V [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate				2.25%
Warehouse Facility VI [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[3]			CAC Warehouse Funding LLC VI
Debt maturity date	[3],[5]			Sep. 30, 2020
Debt financing amount	[3]			$ 75,000,000
Warehouse Facility VI [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate				2.00%
Warehouse Facility VII [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[3]			CAC Warehouse Funding LLC VII
Debt maturity date	[3],[7]			Dec. 01, 2019
Debt financing amount	[3],[4]			$ 150,000,000
Warehouse Facility VII Class A [Member] | Commercial Paper Offered Rate [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate				1.90%
Warehouse Facility VII Class B [Member] | Commercial Paper Offered Rate [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate				2.20%
Term ABS 2015-1 [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[3]			Credit Acceptance Funding LLC 2015-1
Debt maturity date	[3],[5]			Jan. 16, 2017
Debt financing amount	[3]			$ 300,600,000
Term ABS 2015-2 [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[3]			Credit Acceptance Funding LLC 2015-2
Debt maturity date	[3],[5]			Aug. 15, 2017
Debt financing amount	[3]			$ 300,200,000
Term ABS 2016-1 [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[3]			Credit Acceptance Funding LLC 2016-1
Debt maturity date	[3],[5]			Feb. 15, 2018
Debt financing amount				$ 385,000,000	[3]	$ 385,000,000
Term ABS 2016-1 [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate				1.90%
Term ABS 2016-2 [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[3]			Credit Acceptance Funding LLC 2016-2
Debt maturity date	[3],[5]			May 15, 2018
Debt financing amount	[3]			$ 350,200,000
Term ABS 2016-3 [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[3]			Credit Acceptance Funding LLC 2016-3
Debt maturity date	[3],[5]			Oct. 15, 2018
Debt financing amount	[3]			$ 350,000,000
Term ABS 2017-1 [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[3]			Credit Acceptance Funding LLC 2017-1
Debt maturity date	[3],[5]			Feb. 15, 2019
Debt financing amount	[3]			$ 350,000,000
Term ABS 2017-2 [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[3]			Credit Acceptance Funding LLC 2017-2
Debt maturity date	[3],[5]			Jun. 17, 2019
Debt financing amount	[3]			$ 450,000,000
Term ABS 2017-3 [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[3]			Credit Acceptance Funding LLC 2017-3
Debt maturity date	[3],[5]			Oct. 15, 2019
Debt financing amount	[3]			$ 350,000,000
2021 Senior Notes [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary				n/a
Debt maturity date			Feb. 15, 2021	Feb. 15, 2021
Debt financing amount			$ 300,000,000	$ 300,000,000
2023 Senior Notes [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary				n/a
Debt maturity date		Mar. 15, 2023		Mar. 15, 2023
Debt financing amount		$ 250,000,000		$ 250,000,000

[1] The amount of the facility will decrease to $315.0 million on June 22, 2019.

[2] Excludes deferred debt issuance costs of $2.8 million and $2.4 million as of December 31, 2017 and December 31, 2016, respectively, which are included in other assets.

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

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

[5] Represents the revolving maturity date. The outstanding balance will amortize after the revolving maturity date based on the cash flows of the pledged assets.

[6] Represents the revolving maturity date. The outstanding balance will amortize after the revolving maturity date and any amounts remaining on August 18, 2021 will be due on that date.

[7] Represents the revolving maturity date. The outstanding balance will amortize after the revolving maturity date and any amounts remaining on November 30, 2021 will be due on that date.