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Significant Accounting Policies (Details 4) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Goodwill and intangible assets    
Annual goodwill impairment testing date November 30  
Insurance    
Deductible for individual healthcare claims $ 0.3  
Deductible for general liability claims 0.3  
Deductible for workers' compensation 0.5  
Self insurance reserve $ 3.5 $ 4.6