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Concentration of Risk and Enterprise Wide Disclosures - Contract Revenues by Customer (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue, Major Customer [Line Items]
Contract revenues	$ 1,018,484	$ 927,335	$ 868,866
Utah Department of Transportation (“UDOT”) | Customer Concentration Risk
Revenue, Major Customer [Line Items]
Contract revenues	$ 135,496	$ 153,276	$ 140,529
Utah Department of Transportation (“UDOT”) | Customer Concentration Risk | Revenue from Contract with Customer
Revenue, Major Customer [Line Items]
Concentration risk percentage	12.00%	14.80%	14.70%
Texas Department of Transportation (“TXDOT”) | Customer Concentration Risk
Revenue, Major Customer [Line Items]
Contract revenues			$ 103,236
Texas Department of Transportation (“TXDOT”) | Customer Concentration Risk | Revenue from Contract with Customer
Revenue, Major Customer [Line Items]
Concentration risk percentage			10.80%
The Conlan Company
Revenue, Major Customer [Line Items]
Contract receivable	$ 18,700
The Conlan Company | Customer Concentration Risk | Contract Receivables
Revenue, Major Customer [Line Items]
Concentration risk percentage	11.00%

X

- Definition

Amount billed to customers under long-term contracts or programs but not paid. Excludes amounts due that have been withheld because of retainage provisions in a contract.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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