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Debt (Narrative) (Details)
9 Months Ended 12 Months Ended
Nov. 30, 2013
USD ($)
Sep. 30, 2018
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Oct. 31, 2017
USD ($)
Oct. 30, 2017
USD ($)
Dec. 12, 2016
USD ($)
Dec. 11, 2016
USD ($)
Oct. 31, 2013
USD ($)
Debt Instrument [Line Items]                    
Debt instrument, carrying value     $ 518,711,000 $ 507,906,000            
Amortization of deferred loan costs     1,900,000 1,700,000 $ 1,900,000          
Deferred loan costs written off     0 0 $ 2,978,000          
2017 Credit Facility                    
Debt Instrument [Line Items]                    
Unused capacity, commitment fee percentage   0.375%                
Revolving Credit Facility | 2017 Credit Facility                    
Debt Instrument [Line Items]                    
Line of credit facility, accordion feature, increase limit             $ 100,000,000      
Revolving Credit Facility | 2017 Credit Facility | Federal Funds Rate                    
Debt Instrument [Line Items]                    
Basis spread on variable rate   0.50%                
Revolving Credit Facility | 2017 Credit Facility | LIBOR                    
Debt Instrument [Line Items]                    
Basis spread on variable rate   1.00%                
Revolving Credit Facility | 2017 Credit Facility | CDOR Rate                    
Debt Instrument [Line Items]                    
Basis spread on variable rate   1.00%                
Debt Instrument, Redemption, Period Four | Revolving Credit Facility | 2017 Credit Facility                    
Debt Instrument [Line Items]                    
Commitment fee percentage   0.50%                
Debt Instrument, Redemption, Period Five | Revolving Credit Facility | 2017 Credit Facility | Minimum                    
Debt Instrument [Line Items]                    
Commitment fee percentage   0.25%                
Debt Instrument, Redemption, Period Five | Revolving Credit Facility | 2017 Credit Facility | Maximum                    
Debt Instrument [Line Items]                    
Commitment fee percentage   0.375%                
Senior unsecured notes due October 2021                    
Debt Instrument [Line Items]                    
Debt, face amount $ 100,000,000                 $ 300,000,000
Debt instrument, stated interest rate                   6.25%
Debt instrument, issuance price of par, percentage 103.25%                  
Proceeds from issuance of senior debt $ 394,000,000                  
Debt instrument, carrying value     $ 400,000,000 400,000,000            
Senior unsecured notes due October 2021 | Debt Instrument, Redemption, Period Two                    
Debt Instrument [Line Items]                    
Redemption price percentage     101.563%              
Senior unsecured notes due October 2021 | Debt Instrument, Redemption, Period Three                    
Debt Instrument [Line Items]                    
Redemption price percentage     100.00%              
Senior secured revolving credit facility                    
Debt Instrument [Line Items]                    
Maximum borrowing capacity   $ 20,000,000                
Debt instrument, carrying value     $ 119,000,000 $ 108,446,000            
Percentage of borrowing base   10.00%                
Senior secured revolving credit facility | Minimum                    
Debt Instrument [Line Items]                    
Springing fixed charge coverage ratio   1                
Fixed charge coverage ratio consecutive days threshold   60 days                
Senior secured revolving credit facility | 2017 Credit Facility                    
Debt Instrument [Line Items]                    
Current borrowing capacity     299,400,000              
Debt instrument, carrying value   $ 119,000,000                
Senior secured revolving credit facility | Revolving Credit Facility                    
Debt Instrument [Line Items]                    
Current borrowing capacity             140,000,000      
Maximum borrowing capacity           $ 300,000,000   $ 140,000,000 $ 600,000,000  
Deferred financing costs           $ 2,400,000        
Senior secured revolving credit facility | Foreign Line of Credit                    
Debt Instrument [Line Items]                    
Maximum borrowing capacity             25,000,000      
Senior secured revolving credit facility | Letter of Credit                    
Debt Instrument [Line Items]                    
Maximum borrowing capacity             30,000,000      
Senior secured revolving credit facility | Letter of Credit | 2017 Credit Facility                    
Debt Instrument [Line Items]                    
Outstanding letters of credit     13,100,000              
Remaining borrowing capacity     $ 167,300,000              
Senior secured revolving credit facility | Letter of Credit | Canadian Subsidiaries                    
Debt Instrument [Line Items]                    
Maximum borrowing capacity             $ 3,000,000      
Senior secured revolving credit facility | Debt Instrument, Redemption, Period Two                    
Debt Instrument [Line Items]                    
Senior secured debt to adjusted EBITDA ratio   4                
Debt, weighted average interest rate (percentage)     4.08%              
Senior secured revolving credit facility | Debt Instrument, Redemption, Period Three                    
Debt Instrument [Line Items]                    
Minimum leverage ratio   0.02                
Senior secured debt to adjusted EBITDA ratio   0.015                
Percentage of borrowing base   50.00%                
Senior secured revolving credit facility | Debt Instrument, Redemption, Period One                    
Debt Instrument [Line Items]                    
Minimum leverage ratio   0.0175                
Senior secured debt to adjusted EBITDA ratio   0.0225