Note 2 - Significant Accounting Policies (Details Textual) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Feb. 25, 2019
Depreciation, Total		$ 157,800,000	$ 158,700,000	$ 149,100,000
Amortization, Total		19,500,000	16,800,000	36,000,000.0
Goodwill, Impairment Loss	$ 0
Goodwill, Ending Balance	292,142,000	292,142,000	292,142,000	291,391,000
Capitalized Computer Software, Net, Ending Balance	7,000,000.0	7,000,000.0	8,900,000
Capitalized Computer Software, Amortization		1,900,000
Indefinite-Lived Franchise Rights	7,000,000.0	7,000,000.0	7,000,000.0
Advertising Expense		7,900,000	11,500,000	10,400,000
Capitalized Computer Software, Accumulated Amortization	12,100,000	12,100,000	10,200,000
Other Noncurrent Liabilities [Member] | Financial Guarantee [Member]
Guarantor Obligations, Current Carrying Value	5,200,000	5,200,000	$ 5,100,000
Equity Method Investments	32,000,000.0	32,000,000.0
Rush Truck Centres of Canada Limited [Member]
Equity Method Investment, Ownership Percentage			50.00%		50.00%
Rush Truck Centres of Canada Limited [Member] | Call Option [Member]
Equity Method Investment, Remaining Ownership Percentage Not Yet Acquired					50.00%
Rush Truck Centres of Canada Limited [Member] | Call Option [Member] | Other Assets [Member]
Call Option, Fair Value Disclosure	3,600,000	3,600,000
Computer Software, Intangible Asset [Member]
Capitalized Computer Software, Amortization			$ 1,900,000
Finite-Lived Intangible Asset, Expected Amortization, Year One	1,200,000	1,200,000
Finite-Lived Intangible Assets, Amortization Expense, Year Two	1,200,000	1,200,000
Finite-Lived Intangible Assets, Amortization Expense, Year Three	1,200,000	1,200,000
Finite-Lived Intangible Assets, Amortization Expense, Year Four	1,200,000	1,200,000
Finite-Lived Intangible Assets, Amortization Expense, Year Five	1,200,000	1,200,000
Components of ERP Platform Rquired Replacement [Member]
Capitalized Computer Software, Amortization				19,900,000
Franchise Rights [Member]
Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill)		0
Construction in Progress [Member]
Interest Costs Capitalized		163,500
Vehicles [Member]
Depreciation, Total		119,900,000	$ 120,100,000	$ 114,600,000
Finance Lease, Right-of-Use Asset, after Accumulated Amortization, Total	109,500,000	109,500,000
Finance Lease, Right-of-Use Asset, Accumulated Amortization	$ 40,200,000	$ 40,200,000