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Concentration of Risk and Enterprise Wide Disclosures - Contract Revenues by Customer (Details) - USD ($) $ in Thousands | 12 Months Ended
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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%

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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://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 -Paragraph 21 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13537-108611 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592 + Details
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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 + Details
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