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Segment Information - Summary of segment's internal financial reports (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2018 | [1] | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | [1] | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Contract revenues | $ 446,345 | $ 399,537 | $ 339,676 | $ 345,611 | $ 373,501 | $ 373,502 | $ 356,185 | $ 300,129 | $ 1,531,169 | $ 1,403,317 | $ 1,142,487
Income from operations |  |  |  |  |  |  |  |  | 50,312 | 29,558 | 38,754
T&D [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Contract revenues |  |  |  |  |  |  |  |  | 893,108 | 879,372 | 818,972
Income from operations |  |  |  |  |  |  |  |  | 57,242 | 39,631 | 63,459
C&I [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Contract revenues |  |  |  |  |  |  |  |  | 638,061 | 523,945 | 323,515
Income from operations |  |  |  |  |  |  |  |  | 34,112 | 25,048 | 13,920
General Corporate [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Income from operations |  |  |  |  |  |  |  |  | $ (41,042) | $ (35,121) | $ (38,625)
[1] | Results for the fourth quarter of 2017 include a net 2017 Tax Act benefit of $7.8 million, or $0.48 and $0.47 per basic and diluted share, respectively.

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