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Senior Notes (Detail)
May 08, 2018
USD ($)
Jul. 14, 2010
USD ($)
Contract
Jun. 30, 2019
USD ($)
Sep. 30, 2018
USD ($)
Debt Instrument [Line Items]        
Senior Notes, weighted average interest rate     3.781%  
Carrying value of Senior Notes     $ 513,000,000 $ 513,000,000
Fair value of Senior Notes     532,374,000 518,413,000
Debt issuance costs     5,400,000 6,100,000
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 9 months 18 days    
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     114,374,000 114,413,000
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     400,000,000 400,000,000
Fair value of Senior Notes     $ 418,000,000 $ 404,000,000
Debt instrument, maturity date May 15, 2026      
Debt Instrument, Interest Rate, Stated Percentage 5.25%