Debt (Schedule of General Information of Financing Transaction) (Details) - USD ($)				9 Months Ended
		Mar. 30, 2015	Jan. 22, 2014	Sep. 30, 2018		Aug. 06, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Debt facility financing amount				$ 3,901,700,000			$ 3,093,000,000
Line of Credit Two Thousand Nineteen [Member]
Debt Instrument [Line Items]
Financing Amount				$ 315,000,000
Revolving Secured Line of Credit [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary				n/a
Line of credit maturity date				Jun. 22, 2021
Financing Amount	[1]			$ 350,000,000
Debt facility financing amount	[2]			$ 0			13,900,000
Revolving Secured Line of Credit [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on variable rate				1.875%
Revolving Secured Line of Credit [Member] | Prime Rate [Member]
Debt Instrument [Line Items]
Basis spread on variable rate				0.875%
Warehouse Facility II [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[3]			CAC Warehouse Funding Corp. II
Debt maturity date	[3],[4]			Dec. 20, 2020
Debt facility financing amount	[3],[4]			$ 400,000,000
Warehouse Facility II [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	[3],[5]			1.75%
Warehouse Facility IV [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[3]			CAC Warehouse Funding LLC IV
Debt maturity date	[3],[4]			Apr. 30, 2020
Debt facility financing amount				$ 250,000,000	[3],[4]		$ 100,000,000
Warehouse Facility IV [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	[3],[5]			2.25%
Warehouse Facility V [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[3]			CAC Warehouse Funding LLC V
Debt maturity date	[3],[6]			Aug. 17, 2021
Debt facility financing amount	[3],[4]			$ 100,000,000
Warehouse Facility V [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	[3],[5]			1.90%
Warehouse Facility VI [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[3]			CAC Warehouse Funding LLC VI
Debt maturity date	[3],[4]			Sep. 30, 2020
Debt facility financing amount	[3]			$ 75,000,000
Warehouse Facility VI [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	[3]			2.00%
Warehouse Facility VII [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[3]			CAC Warehouse Funding LLC VII
Debt maturity date	[3],[5]			Dec. 01, 2019
Debt facility financing amount	[3]			$ 150,000,000
Warehouse Facility VII Class A [Member] | Commercial Paper Offered Rate [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	[3],[5]			1.90%
Warehouse Facility VII Class B [Member] | Commercial Paper Offered Rate [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	[3],[5]			2.20%
Term ABS 2015-2 [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[3]			Credit Acceptance Funding LLC 2015-2
Debt maturity date	[3],[4]			Aug. 15, 2017
Debt facility 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],[4]			Feb. 15, 2018
Debt facility financing amount	[3],[4]			$ 385,000,000
Term ABS 2016-1 [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	[3],[5]			1.95%
Term ABS 2016-2 [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[3]			Credit Acceptance Funding LLC 2016-2
Debt maturity date	[3],[4]			May 15, 2018
Debt facility 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],[4]			Oct. 15, 2018
Debt facility 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],[4]			Feb. 15, 2019
Debt facility 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],[4]			Jun. 17, 2019
Debt facility 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],[4]			Oct. 15, 2019
Debt facility financing amount	[3]			$ 350,000,000
Term ABS 2018-1 [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[3]			Credit Acceptance Funding LLC 2018-1
Debt maturity date	[3],[4]			Feb. 17, 2020
Debt facility financing amount	[3]			$ 500,000,000
Term ABS 2018-2 [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[3]			Credit Acceptance Funding LLC 2018-2
Debt maturity date	[3],[4]			May 15, 2020
Debt facility financing amount	[3]			$ 450,000,000
Term ABS 2018-3 [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary	[3]			Credit Acceptance Funding LLC 2018-3
Debt maturity date	[3],[4]			Aug. 17, 2020
Debt facility financing amount	[3]			$ 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 facility 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 facility financing amount		$ 250,000,000.0		$ 250,000,000
Mortgages [Member]
Debt Instrument [Line Items]
Wholly-owned Subsidiary				Chapter 4 Properties, LLC
Debt maturity date				Aug. 06, 2023
Debt facility financing amount				$ 12,000,000		$ 12,000,000.0
Mortgages [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	[3],[5]			1.50%

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

[2] Warehouse facilities and Term ABS.

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

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

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

[6] 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.