Fair Value Measurements - Contingent Consideration Liability (Details) $ in Thousands		1 Months Ended		12 Months Ended
	May 26, 2017 USD ($)	Dec. 31, 2017 USD ($) item	Mar. 31, 2015 USD ($)	Dec. 31, 2017 USD ($) item	Dec. 31, 2015 USD ($)	May 31, 2017 USD ($)
Rollforward of contingent consideration liability level three fair value measurements
Change in fair value of contingent consideration liability during year				$ (484)	$ (6,722)
FGC
Additional information
Contingent consideration in cash	$ 1,500					$ 1,500
Fair value of the contingent consideration	1,200					$ 1,200
Contingent consideration credited to non-operating income		$ 500
Payment to Q3C sellers for meeting performance targets	$ 33,000
Q3 Contracting
Additional information
Payment to Q3C sellers for meeting performance targets			$ 5,000
Contingent Consideration Liability | Significant Unobservable Inputs (Level 3)
Rollforward of contingent consideration liability level three fair value measurements
Florida Gas Contractors acquisition				1,200
Change in fair value of contingent consideration liability during year				(484)
Balance at the end of the period		$ 716		$ 716
Additional information
Number of unobservable inputs | item		2		2
Minimum probability of acquired entity meeting contractual operating performance target (as a percent)				33.00%
Maximum probability of acquired entity meeting contractual operating performance target (as a percent)				100.00%