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Concentration of Risk and Enterprise Wide Disclosures - Contract Revenues by Customer (Details) - Customer Concentration Risk - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
California Department of Transportation (“Caltrans”)
Revenue, Major Customer [Line Items]
Contract revenues	$ 140,529	$ 79,421
California Department of Transportation (“Caltrans”) | Sales Revenue, Net
Revenue, Major Customer [Line Items]
Concentration risk percentage	14.70%	11.50%
Texas Department of Transportation (“TXDOT”)
Revenue, Major Customer [Line Items]
Contract revenues	$ 103,236	$ 85,224	$ 84,129
Texas Department of Transportation (“TXDOT”) | Sales Revenue, Net
Revenue, Major Customer [Line Items]
Concentration risk percentage	10.80%	12.40%	13.50%
California Department of Transportation (“Caltrans”)
Revenue, Major Customer [Line Items]
Contract revenues		$ 88,627	$ 96,470
California Department of Transportation (“Caltrans”) | Sales Revenue, Net
Revenue, Major Customer [Line Items]
Concentration risk percentage		12.80%	15.50%

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/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6351-108592

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13531-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6404-108592

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13537-108611

+ Details

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X

- Definition

Costs incurred and are directly related to generating contract revenues.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.2(e))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

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X

- Definition

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.

+ Details

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