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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
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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%

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- 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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- 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 + Details
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