Summary of Significant Accounting Policies - Foreign Operations (Details) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($) $ / shares	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) item $ / shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Basis of Presentation
Assets	$ 1,830,465				$ 1,594,147				$ 1,830,465	$ 1,594,147
Revenue	789,778	$ 865,064	$ 789,929	$ 661,558	877,670	$ 908,902	$ 648,787	$ 504,119	3,106,329	2,939,478	$ 2,379,995
Income (loss) before tax of Canadian operations									10,270	2,356	(272)
Cash concentration
Cash and cash equivalents	120,286				151,063				$ 120,286	151,063
Collective bargaining agreements
Percentage of labor force subject to collective bargaining agreements									41.00%
Number of collective bargaining agreements | item									53
Number of collective bargaining agreements requiring renegotiation during the year | item									25
Number of years without work stoppages									20 years
Insurance
Self- insurance amount per claim									$ 500	500
Self-insurance reserve	39,300				42,800				39,300	42,800
Accounts receivable
Allowance for doubtful accounts	$ 400				1,700				400	1,700
Estimated net impact of change in estimate
Decrease in in net income from revision in contract estimates									$ (3,400)
EPS impact to year | $ / shares	$ (0.07)								$ (0.07)
Gross profit	$ 89,514	$ 108,421	$ 80,531	$ 52,460	$ 103,253	$ 106,505	$ 71,419	$ 44,560	$ 330,926	$ 325,737	$ 278,434