CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS’ AND MEMBERS’ EQUITY - USD ($) $ in Thousands	Members' Equity	Common Stock Class A Common Stock	Common Stock Class B Common Stock	Additional Paid-in Capital	Accumulated Earnings (Deficit)	Treasury Stock	Non controlling Interest	Class A Common Stock	Total
Balance at beginning of year at Dec. 31, 2014	$ 60,620								$ 60,620
Changes in Members' Equity
Member contributions	8,162								8,162
Purchase of member units	(5)								(5)
Issuance of membership units in exchange for notes receivable, includes accrued interest	5,766								5,766
Notes receivable from unit-holders, includes accrued interest	(5,766)								(5,766)
Unit-based compensation expense	64								64
Net Income prior to the Reorganization									(1,373)
Net income (loss)	(1,373)								(1,373)
Balance at end of year at Dec. 31, 2015	67,468								67,468
Changes in Members' Equity
Unit-based compensation expense	127								127
Additional members’ equity related to accrued interest on notes receivable that were exchanged for membership units	(327)								(327)
Accrued interest related to notes receivable that were exchanged for membership units	(327)								(327)
Proceeds from pay down of promissory note related to membership units	948								948
Net Income prior to the Reorganization									2,803
Net income (loss)	2,803								2,803
Balance at end of year at Dec. 31, 2016	71,346								71,346
Changes in Members' Equity
Unit-based compensation expense	43								43
Additional members’ equity related to accrued interest on notes receivable that were exchanged for membership units	(84)								(84)
Accrued interest related to notes receivable that were exchanged for membership units	(84)								(84)
Proceeds from pay down of promissory note related to membership units	3,808								3,808
Net Income prior to the Reorganization	3,665								3,665
Effect of the Reorganization	$ (78,862)	$ 101		$ 77,256			$ 125,056		123,551
Effect of the Reorganization (in shares)		10,100,000	32,366,000
Deferred tax asset and payables related to parties pursuant to Tax Receivable Agreement from the Reorganization				(19,149)					(19,149)
Effect of November offering		$ 81		66,352			(21,969)		44,464
Effect of November offering (in shares)		8,050,000	(5,050,000)					75,130
Deferred tax asset and payables related to parties pursuant to Tax Receivable Agreement from the November Offering				(12,928)					(12,928)
Exchange of B shares for A shares		$ 8		1,997			(2,005)
Exchange of B shares for A shares (in shares)		785,000	(785,000)
Deferred tax asset and payables related to parties pursuant to Tax Receivable Agreement from the exchange of B shares for A shares				(2,755)					$ (2,755)
Stock option exercises				261		$ (261)
Options exercised (in shares)		75,000				16,000			91,484
Issuance of B shares in connection with the acquisition of the assets of Railtronix				4,507					$ 4,507
Issuance of B shares in connection with the acquisition of the assets of Railtronix (in shares)			280,000
Stock-based compensation subsequent to the Reorganization				4,738					4,738
Additional members’ equity related to accrued interest on notes receivable that were exchanged for membership units subsequent to the Reorganization				28					28
Accrued interest related to notes receivable that were exchanged for membership units subsequent to the Reorganization				(28)					(28)
Proceeds from pay down of promissory note and interest related to membership units subsequent to the Reorganization				1,448					1,448
Net income subsequent to the Reorganization					$ 3,636		15,186		18,822
Net income (loss)									22,487
Balance at end of year at Dec. 31, 2017		$ 190		$ 121,727	$ 3,636	$ (261)	$ 116,268		$ 241,560
Balance at end of year (in shares) at Dec. 31, 2017		19,010,000	26,811,000			16,000