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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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Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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