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Concentration of Risk and Enterprise Wide Disclosures - Contract Revenues by Customer (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Major Customer [Line Items]
Contract revenues	$ 927,335	$ 868,866	$ 648,065
California Department of Transportation (“Caltrans”) | Customer Concentration Risk
Revenue, Major Customer [Line Items]
Contract revenues	$ 153,276	$ 140,529	$ 79,421
California Department of Transportation (“Caltrans”) | Customer Concentration Risk | Revenue from Contract with Customer
Revenue, Major Customer [Line Items]
Concentration risk percentage	14.80%	14.70%	11.50%
Texas Department of Transportation (“TXDOT”) | Customer Concentration Risk
Revenue, Major Customer [Line Items]
Contract revenues		$ 103,236	$ 85,224
Texas Department of Transportation (“TXDOT”) | Customer Concentration Risk | Revenue from Contract with Customer
Revenue, Major Customer [Line Items]
Concentration risk percentage		10.80%	12.40%
California Department of Transportation (“Caltrans”) | Customer Concentration Risk
Revenue, Major Customer [Line Items]
Contract revenues			$ 88,627
California Department of Transportation (“Caltrans”) | Customer Concentration Risk | Revenue from Contract with Customer
Revenue, Major Customer [Line Items]
Concentration risk percentage			12.80%

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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6351-108592

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13531-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13537-108611

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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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.2(a),(d))
-URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227

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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.

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No definition available.

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