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Financial Instruments (Narrative) (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Dec. 18, 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 2.55              
Interest coverage ratio 9.50              
Credit Agreement [Member]                
Debt Instrument [Line Items]                
Interest rate 3.49% 3.40%            
Amount of credit facility           $ 685,000   $ 550,000
Amount of credit facility outstanding $ 506,000              
Additional amount that can be borrowed on facility $ 179,000              
LIBOR spread 1.50%              
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     3.61% 2.11%        
LIBOR rate     1.79%          
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