Debt (Schedule Of General Information Of Financing Transaction) (Details) - USD ($)				12 Months Ended
		Mar. 30, 2015	Jan. 22, 2014	Dec. 31, 2018		Dec. 31, 2017
Revolving Secured Line Of Credit [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary				n/a
Debt maturity date				Jun. 22, 2021
Financing Amount	[1]			$ 350,000,000
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	[2]			CAC Warehouse Funding Corp. II
Debt maturity date	[3]			Dec. 20, 2020
Debt financing amount				$ 400,000,000	[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	[2]			CAC Warehouse Funding LLC IV
Debt maturity date	[3]			Apr. 30, 2020
Debt financing amount				$ 250,000,000	[4]	100,000,000
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	[2]			CAC Warehouse Funding LLC V
Debt maturity date	[5]			Aug. 17, 2021
Debt financing amount				$ 100,000,000	[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				1.90%
Warehouse Facility VI [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[2]			CAC Warehouse Funding LLC VI
Debt maturity date	[3]			Sep. 30, 2020
Debt financing amount				$ 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	[2]			CAC Warehouse Funding LLC VII
Debt maturity date	[6]			Dec. 17, 2020
Debt financing amount				$ 150,000,000	[4]	150,000,000
Warehouse Facility VII [Member] | Commercial Paper Offered Rate [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate				2.00%
Term ABS 2016-1 [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[2]			Credit Acceptance Funding LLC 2016-1
Debt maturity date	[3]			Feb. 15, 2018
Debt financing amount				$ 385,000,000	[4]	$ 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				1.95%
Term ABS 2016-2 [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[2]			Credit Acceptance Funding LLC 2016-2
Debt maturity date	[3]			May 15, 2018
Debt financing amount				$ 350,200,000
Term ABS 2016-3 [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[2]			Credit Acceptance Funding LLC 2016-3
Debt maturity date	[3]			Oct. 15, 2018
Debt financing amount				$ 350,000,000
Term ABS 2017-1 [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[2]			Credit Acceptance Funding LLC 2017-1
Debt maturity date	[3]			Feb. 15, 2019
Debt financing amount				$ 350,000,000
Term ABS 2017-2 [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[2]			Credit Acceptance Funding LLC 2017-2
Debt maturity date	[3]			Jun. 17, 2019
Debt financing amount				$ 450,000,000
Term ABS 2017-3 [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[2]			Credit Acceptance Funding LLC 2017-3
Debt maturity date	[3]			Oct. 15, 2019
Debt financing amount				$ 350,000,000
Term ABS 2018-1 [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[2]			Credit Acceptance Funding LLC 2018-1
Debt maturity date	[3]			Feb. 17, 2020
Debt financing amount				$ 500,000,000
Term ABS 2018-2 [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[2]			Credit Acceptance Funding LLC 2018-2
Debt maturity date	[3]			May 15, 2020
Debt financing amount				$ 450,000,000
Term ABS 2018-3 [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[2]			Credit Acceptance Funding LLC 2018-3
Debt maturity date	[3]			Aug. 17, 2020
Debt financing amount				$ 398,300,000
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
Mortgage [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[2]			Chapter 4 Properties, LLC
Debt maturity date				Aug. 06, 2023
Debt financing amount				$ 12,000,000
Mortgage [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate				1.50%
Mortgage [Member] | Commercial Paper Offered Rate [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate

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

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

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

[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 and any amounts remaining on August 17, 2023 will be due on that date.

[6] Represents the revolving maturity date. The outstanding balance will amortize after the revolving maturity date and any amounts remaining on December 17, 2022 will be due on that date.