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Financial Instruments (Narrative) (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Nov. 28, 2017
Nov. 27, 2017
Nov. 07, 2017
Jun. 18, 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 2.23              
Interest coverage ratio 10.73              
Credit Agreement [Member]                
Debt Instrument [Line Items]                
Interest rate 3.58% 3.40%            
Amount of credit facility         $ 685,000     $ 550,000
Amount of credit facility outstanding $ 511,000              
Additional amount that can be borrowed on facility $ 174,000              
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     2.11%     3.61%    
LIBOR rate           2.09%    
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%              
Capital Lease Obligations [Member]                
Debt Instrument [Line Items]                
Interest rate 5.00%              
Maturity date Dec. 31, 2022