Segment Information - Summary of segment's internal financial reports (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Contract revenues	$ 513,051	$ 448,776	$ 1,031,521	$ 916,870
Income from operations	19,342	9,410	35,750	19,036
T&D [Member]
Segment Reporting Information [Line Items]
Contract revenues	276,759	255,910	536,029	528,458
Income from operations	23,838	16,050	41,802	30,980
C&I [Member]
Segment Reporting Information [Line Items]
Contract revenues	236,292	192,866	495,492	388,412
Income from operations	8,958	4,512	18,270	9,570
General Corporate [Member]
Segment Reporting Information [Line Items]
Income from operations	$ (13,454)	$ (11,152)	$ (24,322)	$ (21,514)