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Note 2 - Significant Accounting Policies (Details Textual) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Feb. 25, 2019
Depreciation, Total		$ 158,700,000	$ 149,100,000	$ 140,300,000
Amortization, Total		16,800,000	36,000,000.0	17,600,000
Goodwill, Impairment Loss	$ 0
Capitalized Computer Software, Net, Ending Balance	8,900,000	8,900,000	10,800,000
Capitalized Computer Software, Amortization		1,900,000	21,700,000
Indefinite-Lived Franchise Rights	7,000,000.0	7,000,000.0	7,000,000.0
Advertising Expense		11,500,000	10,400,000	9,500,000
Capitalized Computer Software, Accumulated Amortization	10,200,000	10,200,000	8,300,000
Other Noncurrent Liabilities [Member] \| Financial Guarantee [Member]
Guarantor Obligations, Current Carrying Value	$ 5,100,000	$ 5,100,000
Rush Truck Centres of Canada Limited [Member]
Equity Method Investment, Ownership Percentage	50.00%	50.00%			50.00%
Rush Truck Centres of Canada Limited [Member] \| Other Assets [Member]
Equity Method Investments	$ 25,700,000	$ 25,700,000
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]
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	1,900,000	1,900,000
Finite-Lived Intangible Assets, Amortization Expense, Year Two	1,900,000	1,900,000
Finite-Lived Intangible Assets, Amortization Expense, Year Three	1,900,000	1,900,000
Finite-Lived Intangible Assets, Amortization Expense, Year Four	1,900,000	1,900,000
Finite-Lived Intangible Assets, Amortization Expense, Year Five	1,900,000	1,900,000
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		387,800
Vehicles [Member]
Depreciation, Total		120,100,000	$ 114,600,000	$ 107,900,000
Finance Lease, Right-of-Use Asset	85,800,000	85,800,000
Finance Lease, Right-of-Use Asset, Accumulated Amortization	$ 43,000,000.0	$ 43,000,000.0