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Financial Instruments (Narrative) (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 16, 2019
Dec. 18, 2018
USD ($)
Nov. 28, 2017
USD ($)
Nov. 27, 2017
USD ($)
Nov. 07, 2017
USD ($)
May 06, 2016
USD ($)
Apr. 08, 2016
USD ($)
Debt Instrument [Line Items]                    
Principal payments due in 2022 $ 424,000                  
Interest paid $ 17,400 $ 18,800 $ 16,000              
Interest rate 5.50% 5.50%                
Finance lease obligation $ 24,535 $ 25,900                
Maximum leverage ratio allowed 3.50                  
Minimum interest coverage ratio required 1.00                  
Leverage ratio 1.35                  
Interest coverage ratio 14.21                  
Credit Agreement [Member]                    
Debt Instrument [Line Items]                    
Amount of credit facility               $ 685,000   $ 550,000
Amount of credit facility outstanding $ 424,000                  
Additional amount that can be borrowed on facility $ 261,000                  
LIBOR spread 1.375%                  
Interest rate at end of period 3.43% 3.69%                
Credit Agreement [Member] | Interest Rate Swap [Member]                    
Debt Instrument [Line Items]                    
Borrowings, revolving credit facility             $ 300,000   $ 120,000  
Notional amount         $ 350,000 $ 350,000        
Fixed interest rate in swap       3.485%   2.11%        
LIBOR rate       1.74%            
Amount paid to terminate agreement                 $ 5,200  
Amount received from terminate agreement             $ 6,300      
Credit Agreement [Member] | Minimum [Member]                    
Debt Instrument [Line Items]                    
LIBOR spread 1.25%                  
Credit Agreement [Member] | Maximum [Member]                    
Debt Instrument [Line Items]                    
LIBOR spread 1.75%                  
Finance obligations [Member]                    
Debt Instrument [Line Items]                    
Maturity date Dec. 31, 2022