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Financial risk management (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Interest Rate Risk [Member]      
Financial risk management [Abstract]      
Forecasted increase in benchmark interest rate 0.25%    
Forecasted loss on cash flow hedges due to increase in benchmark interest rate $ (228) $ (2,563) $ (1,795)
Forecasted increase in hedging reserves due to increase in benchmark interest rate $ 37,767 $ 37,290 $ 41,702
Interest Rate Risk [Member] | Euros [Member]      
Financial risk management [Abstract]      
Maturity date 2030    
Interest Rate Risk [Member] | Euros [Member] | Bottom of Range [Member]      
Financial risk management [Abstract]      
Percentage of notional amount of debt hedged 87.00%    
Interest rate 3.20%    
Interest Rate Risk [Member] | Euros [Member] | Top of Range [Member]      
Financial risk management [Abstract]      
Percentage of notional amount of debt hedged 100.00%    
Interest rate 4.87%    
Interest Rate Risk [Member] | U.S. Dollars [Member]      
Financial risk management [Abstract]      
Maturity date 2032    
Interest Rate Risk [Member] | U.S. Dollars [Member] | Bottom of Range [Member]      
Financial risk management [Abstract]      
Percentage of notional amount of debt hedged 70.00%    
Interest rate 2.32%    
Interest Rate Risk [Member] | U.S. Dollars [Member] | Top of Range [Member]      
Financial risk management [Abstract]      
Percentage of notional amount of debt hedged 100.00%    
Interest rate 5.27%    
Currency Risk [Member]      
Financial risk management [Abstract]      
Net Euro exposure percentage for next fiscal year 100.00%    
Net Euro exposure percentage for following fiscal period 75.00%