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Revolving Line of Credit - Additional Information (Detail)
3 Months Ended 12 Months Ended
Sep. 30, 2017
USD ($)
Sep. 30, 2017
USD ($)
Jun. 30, 2017
USD ($)
Line of Credit Facility [Line Items]      
Line of credit, current borrowing capacity     $ 500,000,000
Line of credit, option to increase borrowing capacity     $ 100,000,000
Revolving credit facility, expiration date   Dec. 30, 2019  
Credit facility restrictive covenant, minimum fixed charge ratio 2.5 2.5  
Credit facility restrictive covenant, maximum consolidated leverage ratio 3.0 3.0  
Credit facility restrictive covenant, maximum consolidated leverage ratio step up 3.5 3.5  
Borrowings outstanding $ 361,000,000 $ 361,000,000  
Interest rate of borrowings outstanding 2.365% 2.365%  
Long-term debt      
Line of Credit Facility [Line Items]      
Borrowings outstanding $ 350,000,000 $ 350,000,000  
Federal Fund Rate      
Line of Credit Facility [Line Items]      
Debt instrument basis spread on variable rate 0.50%    
LIBOR      
Line of Credit Facility [Line Items]      
Debt instrument basis spread on variable rate 1.00%    
LIBOR | Minimum      
Line of Credit Facility [Line Items]      
Debt instrument basis spread on variable rate 1.00%    
LIBOR | Maximum      
Line of Credit Facility [Line Items]      
Debt instrument basis spread on variable rate 1.875%    
Base Rate | Minimum      
Line of Credit Facility [Line Items]      
Debt instrument basis spread on variable rate 0.00%    
Base Rate | Maximum      
Line of Credit Facility [Line Items]      
Debt instrument basis spread on variable rate 0.875%