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Acquisitions (Details Textual) - USD ($)
$ in Thousands
 1 Months Ended 12 Months Ended
Apr. 13, 2015
Oct. 28, 2016
Nov. 24, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Business Acquisition [Line Items]            
Payments to Acquire Businesses, Net of Cash Acquired       $ 0 $ 12,056 $ 13,087
Other Nonoperating Income         $ 1,400  
Other Nonoperating Expense       2,300    
E.S. Boulos Company [Member]            
Business Acquisition [Line Items]            
Business Combination, Consideration Transferred, Total $ 11,400          
High Country Line Construction Inc [Member]            
Business Acquisition [Line Items]            
Business Combination, Consideration Transferred, Total     $ 1,700      
Business Acquisition, Transaction Costs     200      
Finite-lived Intangible Assets Acquired     300      
High Country Line Construction Inc [Member] | Goodwill [Member]            
Business Acquisition [Line Items]            
Goodwill, Acquired During Period     $ 200      
Western Pacific Enterprises [Member]            
Business Acquisition [Line Items]            
Business Acquisition, Transaction Costs       400    
Goodwill, Acquired During Period       200    
Payments to Acquire Businesses, Net of Cash Acquired   $ 12,100        
Business Combination, Contingent Consideration, Liability       2,200    
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability   800        
Escrow Deposit   $ 1,900        
Finite-lived Intangible Assets Acquired       $ 500