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Senior Notes - Additional Information (Detail)
Jul. 14, 2010
USD ($)
Contract
May. 07, 2008
USD ($)
Contract
Dec. 31, 2015
USD ($)
Sep. 30, 2015
USD ($)
Debt Instrument [Line Items]        
Senior Notes, weighted average interest rate     1.756%  
Carrying value of Senior Notes     $ 376,000,000 $ 376,000,000
Fair value of Senior Notes     $ 396,400,000 $ 401,600,000
May 2008 senior notes        
Debt Instrument [Line Items]        
Senior Notes issued in a private placement to a group of institutional investors   $ 275,000,000    
Number of series of Senior Notes issued | Contract   4    
Senior Notes, weighted average interest rate   7.20%    
Senior Notes, weighted average maturity (in years)   10 years    
May 2008 senior notes | Minimum        
Debt Instrument [Line Items]        
Senior Notes, maturity (in years)   5 years    
May 2008 senior notes | Maximum        
Debt Instrument [Line Items]        
Senior Notes, maturity (in years)   10 years    
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 5.20%      
Senior Notes, weighted average maturity (in years) 8 years      
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