Acquisitions - Allocation of Purchase Price (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended								6 Months Ended	9 Months Ended	12 Months Ended
	May 01, 2017	Jul. 15, 2016	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Acquisition
Working capital adjustment											$ 5,417
Payable to former shareholder														$ 5,487
Allocation of the purchase price paid to the fair value of the net assets for acquisition
Goodwill			$ 241,006				$ 241,006						$ 241,006	241,006
Revenues			151,825	$ 124,832	$ 163,446	$ 83,964	137,969	$ 125,339	$ 139,371	$ 72,248			524,067	474,927	$ 416,268
Pre-tax operating losses			19,748	$ 11,069	$ 28,080	$ (3,138)	$ 21,451	$ 15,081	$ 22,354	$ (5,971)			55,759	$ 52,915	63,696
Maximum
Acquisition
Percentage of additional earnout payment							50.00%							50.00%
Henderson
Allocation of the purchase price paid to the fair value of the net assets for acquisition
Earnout			(352)				$ (529)						(352)	$ (529)
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
Earnout consideration amended period												2 years
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
Prepaid and other current assets		705
Property and equipment		5,821
Goodwill		77,354
Intangible assets		77,800
Other assets, long-term		219
Accounts payable and other current liabilities		(3,881)
Floor plan obligations		(13,479)
Earnout		(10,200)	$ (2,200)				$ (3,100)						$ (2,200)	(3,100)
Total		181,344
Amortization period of goodwill, for income tax purpose													15 years
Pro forma Information
Net sales															490,243
Net income															$ 45,983
Earnings per common share assuming dilution attributable to common shareholders															$ 2.00
Revenues since acquisition															$ 65,044
Pre-tax operating income (losses) since acquisition															$ (1,397)
Dejana | Revolving credit facility
Acquisition
Short-term borrowings (repayments), net		$ 20,000
Arrowhead Equipment, Inc. [Member]
Acquisition
Total consideration	$ 7,385
Transactions expenses														343
Allocation of the purchase price paid to the fair value of the net assets for acquisition
Accounts receivable	852
Inventories	1,547
Prepaid and other current assets	6
Property and equipment	624
Goodwill	2,720
Intangible assets	2,700
Accounts payable and other current liabilities	(957)
Unfavorable lease	(107)
Total	$ 7,385
Amortization period of goodwill, for income tax purpose												15 years
Pro forma Information
Revenues since acquisition														7,964
Pre-tax operating income (losses) since acquisition														$ 607