Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Amount	$ 339,676	$ 685,287
Percent	100.00%	100.00%
Contract Type Fixed price [Member]
Amount	$ 167,669	$ 336,259
Percent	49.40%	49.10%
Contract Type Unit Price [Member]
Amount	$ 57,039	$ 112,365
Percent	16.80%	16.40%
Contract Type T&E [Member]
Amount	$ 77,015	$ 166,817
Percent	22.70%	24.30%
Contract Type Other [Member]
Amount	$ 37,953	$ 69,846
Percent	11.10%	10.20%
T&D [Member]
Amount	$ 196,925	$ 413,311
Percent	100.00%	100.00%
T&D [Member] | Contract Type Fixed price [Member]
Amount	$ 77,230	$ 156,665
Percent	39.20%	37.90%
T&D [Member] | Contract Type Unit Price [Member]
Amount	$ 41,631	$ 87,307
Percent	21.10%	21.10%
T&D [Member] | Contract Type T&E [Member]
Amount	$ 68,073	$ 148,418
Percent	34.60%	35.90%
T&D [Member] | Contract Type Other [Member]
Amount	$ 9,991	$ 20,921
Percent	5.10%	5.10%
C&I [Member]
Amount	$ 142,751	$ 271,976
Percent	100.00%	100.00%
C&I [Member] | Contract Type Fixed price [Member]
Amount	$ 90,439	$ 179,594
Percent	63.40%	66.00%
C&I [Member] | Contract Type Unit Price [Member]
Amount	$ 15,408	$ 25,058
Percent	10.80%	9.20%
C&I [Member] | Contract Type T&E [Member]
Amount	$ 8,942	$ 18,399
Percent	6.30%	6.80%
C&I [Member] | Contract Type Other [Member]
Amount	$ 27,962	$ 48,925
Percent	19.50%	18.00%