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Financial Instruments (Narrative) (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2018
Sep. 17, 2018
Dec. 31, 2017
Nov. 28, 2017
Nov. 27, 2017
Nov. 07, 2017
May 06, 2016
Apr. 08, 2016
Debt Instrument [Line Items]                
Maximum leverage ratio allowed 3.75              
Minimum interest coverage ratio required 3.00              
Leverage ratio 2.05              
Interest coverage ratio 11.50              
Credit Agreement [Member]                
Debt Instrument [Line Items]                
Interest rate 3.61%   3.40%          
Amount of credit facility           $ 685,000   $ 550,000
Amount of credit facility outstanding $ 504,000              
Additional amount that can be borrowed on facility $ 181,000              
LIBOR spread 1.50%              
Credit Agreement [Member] | Interest Rate Current 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   3.61%   2.11%        
LIBOR rate   2.16%            
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 lease obligation [Member]                
Debt Instrument [Line Items]                
Interest rate 5.00%              
Maturity date Dec. 31, 2022