Acquisitions - Allocation of Purchase Price (Details) - USD ($) $ / shares in Units, $ in Thousands		6 Months Ended	12 Months Ended
	Jul. 15, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Acquisition
Working capital adjustment		$ 5,417
Short-term borrowings (repayments), net					$ (13,000)
Allocation of the purchase price paid to the fair value of the net assets for acquisition
Goodwill		238,286	$ 238,286	$ 160,932
Trynex
Allocation of the purchase price paid to the fair value of the net assets for acquisition
Earnout				(1,606)
Henderson
Acquisition
Total consideration					98,511
Working capital adjustment					4,688
Payable to former shareholder				3,340
Transaction expenses related to acquisition					1,815
Allocation of the purchase price paid to the fair value of the net assets for acquisition
Cash and cash equivalents					3,950
Accounts receivable					14,951
Inventories					16,308
Refundable income taxes paid					1,206
Deferred income taxes, current					823
Other current assets					876
Property and equipment					10,848
Goodwill					47,800
Intangible assets					17,390
Other assets, long-term					74
Accounts payable and other liabilities					(24,083)
Deferred income taxes, long-term					(3,202)
Other liabilities, long-term					(248)
Earnout		(636)	(636)	(761)
Total					86,693
Pro forma Information
Net sales					385,138
Net income					$ 43,191
Earnings per common share assuming dilution attributable to common shareholders					$ 1.90
Receivables					$ 14,951
Dejana
Acquisition
Total consideration	$ 191,544
Working capital adjustment	3,989
Short-term borrowings (repayments), net	130,000
Payment of working capital			1,428
Discount	650
Payment of on hand cash at acquisition	31,994
Transactions expenses			3,422
Adjusted fair value of earn out consideration		10,373	10,373
Subsequent adjustment			173
Maximum earnout		21,487	21,487
Contingent consideration in the form of an earnout capped		26,000	26,000
Fair value of the contingent consideration recognized			10,200
Allocation of the purchase price paid to the fair value of the net assets for acquisition
Accounts receivable	13,509
Inventories	20,017
Truck chassis floor plan inventory	13,479
Refundable income taxes paid	705
Property and equipment	5,821
Goodwill	77,354
Intangible assets	77,800
Other assets, long-term	219
Accounts payable and other liabilities	(3,881)
Floor plan obligations	(13,479)
Earnout	(10,200)	$ (10,373)	(10,373)
Total	181,344
Pro forma Information
Net sales			490,243	517,716
Net income			$ 45,983	$ 45,760
Earnings per common share assuming dilution attributable to common shareholders			$ 2.00	$ 2.01
Revenues since acquisition			$ 65,044
Pre-tax operating income (losses) since acquisition			$ (1,397)
Receivables	13,509
Dejana | Revolving credit facility
Acquisition
Short-term borrowings (repayments), net	$ 20,000