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Segment Information - Narrative (Details) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2018 USD ($) | Sep. 30, 2018 USD ($) | Jun. 30, 2018 USD ($) | Mar. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Sep. 30, 2017 USD ($) | Jun. 30, 2017 USD ($) | Mar. 31, 2017 USD ($) | Dec. 31, 2018 USD ($) segment | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Contract revenues | $ 446,345 | [1] | $ 399,537 | $ 339,676 | $ 345,611 | $ 373,501 | [1] | $ 373,502 | $ 356,185 | $ 300,129 | $ 1,531,169 | $ 1,403,317 | $ 1,142,487
Assets | 748,755 |  |  |  |  | 603,788 |  |  |  |  | $ 748,755 | 603,788 |
Number of Reportable Segments | segment |  |  |  |  |  |  |  |  |  |  | 2 |  |
Canadian Operation [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Assets | $ 20,500 |  |  |  |  | $ 40,000 |  |  |  |  | $ 20,500 | 40,000 |
Canadian contract revenues [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Contract revenues |  |  |  |  |  |  |  |  |  |  | $ 53,800 | $ 84,100 | $ 36,500
[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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