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Derivative instruments (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Dec. 31, 2017
May 31, 2017
Derivative      
Net change in fair value of cash flow hedge $ 1,500    
Interest rate swap 2,253 $ 773  
Gain loss in fair value 0    
Interest Rate Swap      
Derivative      
Interest rate expense 100    
Interest rate swap $ 2,300 $ 800  
LIBOR | Nondesignated | Interest Rate Swap      
Derivative      
Notional amount     $ 100,000