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Concentration of Risk and Enterprise Wide Disclosures - Contract Revenues by Customer (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue, Major Customer [Line Items]
Contract revenues	$ 1,236,043	$ 1,018,484	$ 927,335
Contract receivable	$ 19,807	$ 18,700
Customer Concentration Risk | Contract Receivables
Revenue, Major Customer [Line Items]
Concentration risk percentage	11.00%	11.00%
Utah Department of Transportation (“UDOT”) | Customer Concentration Risk
Revenue, Major Customer [Line Items]
Contract revenues		$ 135,496	$ 153,276
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%

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.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
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X

- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

The aggregate costs related to goods produced and sold and services rendered by an entity during the reporting period. This excludes costs incurred during the reporting period related to financial services rendered and other revenue generating activities.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
-Topic 924
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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