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Senior Notes (Detail)
Dec. 06, 2019
USD ($)
May 08, 2018
USD ($)
Jul. 14, 2010
USD ($)
Contract
Dec. 31, 2019
USD ($)
Sep. 30, 2019
USD ($)
Debt Instrument [Line Items]          
Senior Notes, weighted average interest rate       2.90%  
Carrying value of Senior Notes       $ 835,000,000 $ 485,000,000
Fair value of Senior Notes       881,829,000 514,121,000
Debt issuance costs       11,700,000 5,200,000
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)     10 years    
Net debt to EBITDA ratio maximum     3.00    
Fixed charge coverage ratio minimum     2.50    
Carrying value of Senior Notes       85,000,000 85,000,000
Fair value of Senior Notes       85,892,000 86,121,000
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       442,000,000 428,000,000
Debt instrument, maturity date   May 15, 2026      
Debt Instrument, Interest Rate, Stated Percentage   5.25%      
Senior Notes December Six [Member]          
Debt Instrument [Line Items]          
Senior Notes issued in a private placement to a group of institutional investors $ 350,000,000        
Carrying value of Senior Notes       350,000,000 0
Fair value of Senior Notes       $ 353,937,000 $ 0
Debt instrument, maturity date Jun. 15, 2028        
Debt Instrument, Interest Rate, Stated Percentage 4.00%