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Debt - Additional Information (Details)
12 Months Ended
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Jul. 31, 2017
USD ($)
Jun. 29, 2016
USD ($)
Debt Instrument [Line Items]        
Debt issuance costs $ 1,400,000 $ 1,700,000    
Debt covenant, interest coverage ratio 4.0      
Debt covenant, leverage ratio (no more than) 3.0      
Senior Facilities Agreement        
Debt Instrument [Line Items]        
Debt, use of proceeds, amount permitted to dispose of assets without restriction (up to) $ 25,000,000      
Loan Notes | Loan Notes due 2018        
Debt Instrument [Line Items]        
Debt maximum borrowing capacity       $ 50,000,000
Debt stated interest rate 6.19%      
Loan Notes | Loan Notes due 2021        
Debt Instrument [Line Items]        
Debt stated interest rate 3.67%      
Loan Notes | Loan Notes due 2023        
Debt Instrument [Line Items]        
Debt stated interest rate 4.88%      
Loan Notes | Loan Notes due 2026        
Debt Instrument [Line Items]        
Debt stated interest rate 4.94%      
Revolving credit facility        
Debt Instrument [Line Items]        
Debt maximum borrowing capacity     $ 150,000,000  
Debt issuance costs     1,000,000  
Debt, weighted average interest rate 3.58% 3.05%    
Uncommitted Facilities        
Debt Instrument [Line Items]        
Accordion facility amount     $ 50,000,000