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Senior Notes (Detail)
May 08, 2018
USD ($)
Jul. 14, 2010
USD ($)
Contract
Jun. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Jun. 30, 2017
USD ($)
May 07, 2008
USD ($)
Debt Instrument [Line Items]            
Senior Notes, weighted average interest rate     3.201%      
Carrying value of Senior Notes     $ 506,713,000 $ 243,801,000    
Fair value of Senior Notes     510,679,000 253,157,000    
Debt issuance costs     (6,287,000) (199,000)    
May 2008 Senior Notes            
Debt Instrument [Line Items]            
Senior Notes issued in a private placement to a group of institutional investors           $ 275,000,000
Carrying value of Senior Notes     0 131,000,000    
Fair value of Senior Notes     0 134,250,000    
July 2010 Senior Notes            
Debt Instrument [Line Items]            
Senior Notes issued in a private placement to a group of institutional investors   $ 245,000,000        
Number of series of Senior Notes issued | Contract   4        
Senior Notes, weighted average interest rate   5.60%        
Senior Notes, weighted average maturity (in years)   9 years 10 months        
Net debt to EBITDA ratio maximum   3.00        
Fixed charge coverage ratio minimum   2.50        
Carrying value of Senior Notes     113,000,000 113,000,000    
Fair value of Senior Notes     114,966,000 $ 119,106,000    
July 2010 Senior Notes | Minimum            
Debt Instrument [Line Items]            
Senior Notes, maturity (in years)   6 years        
July 2010 Senior Notes | Maximum            
Debt Instrument [Line Items]            
Senior Notes, maturity (in years)   10 years        
May 2018 Senior Notes            
Debt Instrument [Line Items]            
Senior Notes issued in a private placement to a group of institutional investors $ 400,000,000          
Carrying value of Senior Notes     400,000,000   $ 0  
Fair value of Senior Notes     $ 402,000,000   $ 0  
Debt instrument, maturity date May 15, 2026          
Debt Instrument, Interest Rate, Stated Percentage 5.25%