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Subsequent Event (Details) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Business Acquisition [Line Items]        
Payments to Acquire Businesses, Gross   $ 30,684,000 $ 0 $ 0
Stock option price threshold   $ 19.00    
Subsequent Event [Member]        
Business Acquisition [Line Items]        
Common Stock, Par or Stated Value Per Share $ 1.00      
Business Combination, Contingent Consideration, Asset, Current $ 3.80      
Subsequent Event [Member] | Common Shares        
Business Acquisition [Line Items]        
Payments to Acquire Businesses, Gross $ 0.1378