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Fair value of financial instruments (Details)
$ in Millions
3 Months Ended
Mar. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Mar. 31, 2016
USD ($)
derivative_instrument
Fixed-Rate Debt | Unsecured notes issued in a private placement      
Debt Instrument [Line Items]      
Carrying value $ 78.6 $ 85.7  
Fair Value, Inputs, Level 3 | Fixed-Rate Debt | Unsecured notes issued in a private placement      
Debt Instrument [Line Items]      
Fair value 79.9 $ 88.2  
Not Designated as Hedging Instrument | Foreign Exchange Contract      
Debt Instrument [Line Items]      
Notional value $ 28.6    
Weighted average maturity 1 month    
Designated as Hedging Instrument | Cross currency swap      
Debt Instrument [Line Items]      
Notional value $ 6.3    
Weighted average maturity 6 months    
Designated as Hedging Instrument | Interest Rate Swap      
Debt Instrument [Line Items]      
Notional value     $ 40.0
Weighted average maturity 2 years 15 days    
Number of interest rate swaps | derivative_instrument     2
Prepaid Expenses and Other Current Assets | Designated as Hedging Instrument | Cross currency swap | Fair Value, Inputs, Level 2      
Debt Instrument [Line Items]      
Fair value of contract $ 2.3    
Other Assets | Designated as Hedging Instrument | Interest Rate Swap | Fair Value, Inputs, Level 2      
Debt Instrument [Line Items]      
Fair Value of swap, net $ 0.9