Summary of Significant Accounting Principles - Revenue Recognition (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Revenue, Major Customer [Line Items]
Costs and estimated earnings in excess of billings on uncompleted contracts	$ 15,133		$ 9,217
Billings in excess of costs and estimated earnings on uncompleted contracts	35,865		21,761
Deferred income taxes	83		49
Retained loss	(39,785)		(31,861)
Contract revenues	143,105	$ 136,843
Costs of contract revenues	132,886	121,021
Gross profit	10,219	15,822
Income tax expense	593	1,489
Net (loss) income	$ (7,924)	$ 4,101
Basic (loss) income per share (in dollars per share)	$ (0.27)	$ 0.15
Diluted (loss) income per share (in dollars per share)	$ (0.27)	$ 0.14
Customer One
Revenue, Major Customer [Line Items]
Contract revenues	$ 1,100
Costs And Estimated Earnings In Excess Of Billings On Uncompleted Contracts [Member]
Revenue, Major Customer [Line Items]
Contract with customer, liability, revenue recognized			$ 0