Concentration of Risk and Enterprise-Wide Disclosures (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Concentration Risk [Line Items]
Contract revenues	$ 199,507	$ 125,073	$ 508,597	$ 421,682
Customer concentration risk | Trade and contract retainage receivables
Concentration Risk [Line Items]
Trade and retainage receivables	142,093		$ 142,093		$ 108,375
Concentration risk, percentage			100.00%		100.00%
Customer concentration risk | Contract revenues
Concentration Risk [Line Items]
Contract revenues	$ 199,507	$ 125,073	$ 508,597	$ 421,682
Concentration risk, percentage	100.00%	100.00%	100.00%	100.00%
Customer concentration risk | Federal Government | Trade and contract retainage receivables
Concentration Risk [Line Items]
Trade and retainage receivables	$ 595		$ 595		$ 2,319
Concentration risk, percentage			1.00%		2.00%
Customer concentration risk | Federal Government | Contract revenues
Concentration Risk [Line Items]
Contract revenues	$ 13,564	$ 18,072	$ 36,142	$ 47,170
Concentration risk, percentage	7.00%	15.00%	7.00%	11.00%
Customer concentration risk | State Governments | Trade and contract retainage receivables
Concentration Risk [Line Items]
Trade and retainage receivables	$ 4,279		$ 4,279		$ 916
Concentration risk, percentage			3.00%		1.00%
Customer concentration risk | State Governments | Contract revenues
Concentration Risk [Line Items]
Contract revenues	$ 19,872	$ 7,674	$ 34,213	$ 25,947
Concentration risk, percentage	10.00%	6.00%	7.00%	6.00%
Customer concentration risk | Local Governments | Trade and contract retainage receivables
Concentration Risk [Line Items]
Trade and retainage receivables	$ 40,006		$ 40,006		$ 30,187
Concentration risk, percentage			28.00%		28.00%
Customer concentration risk | Local Governments | Contract revenues
Concentration Risk [Line Items]
Contract revenues	$ 54,654	$ 25,546	$ 150,683	$ 69,298
Concentration risk, percentage	27.00%	20.00%	30.00%	17.00%
Customer concentration risk | Private Companies | Trade and contract retainage receivables
Concentration Risk [Line Items]
Trade and retainage receivables	$ 97,213		$ 97,213		$ 74,953
Concentration risk, percentage			68.00%		69.00%
Customer concentration risk | Private Companies | Contract revenues
Concentration Risk [Line Items]
Contract revenues	$ 111,417	$ 73,781	$ 287,559	$ 279,267
Concentration risk, percentage	56.00%	59.00%	57.00%	66.00%