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Commitments and Contingencies (Details Textual) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Other Commitments [Line Items]      
Outstanding Performance Bonds $ 497.0    
Estimated Cost to Complete Bonded Projects 144.5    
Purchase Commitment, Remaining Minimum Amount Committed 12.3    
Insurance Claims [Member]      
Other Commitments [Line Items]      
Loss Contingency Insurance Expense 29.5 $ 25.6 $ 23.6
Maximum [Member]      
Other Commitments [Line Items]      
Loss Contingency Insurance Policy Deductible for Each Line of Coverage Excluding Wildfire and Health 1.0    
Loss Contingency Insurance Policy Deductible for Wildfire 2.0    
Loss Contingency Health Insurance Deductible For Qualified Individuals $ 0.2