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Derivative instruments (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2018
Dec. 31, 2017
May 31, 2017
Derivative        
Net change in fair value of cash flow hedge $ 400 $ 2,500    
Interest rate swap 3,302 3,302 $ 773  
Gain loss in fair value 0 0    
Interest Rate Swap        
Derivative        
Interest rate expense 0 0    
Interest rate swap $ 3,300 $ 3,300 $ 800  
LIBOR | Nondesignated | Interest Rate Swap        
Derivative        
Notional amount       $ 100,000