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Revolving Line of Credit (Detail) - USD ($)
3 Months Ended
Mar. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Line Of Credit Facility      
Outstanding borrowings $ 0 $ 0  
Revolving Credit Facility | Credit Agreement      
Line Of Credit Facility      
Revolving credit facility, maximum borrowing capacity     $ 400,000,000
Outstanding borrowings 0    
Line of credit facility, unused borrowing capacity $ 400,000,000.0    
Revolving Credit Facility | Credit Agreement | Prime Commercial Lending Rate      
Line Of Credit Facility      
Effective percentage interest rate 3.38%    
Revolving Credit Facility | Credit Agreement | Libor Rate      
Line Of Credit Facility      
Effective percentage interest rate 2.12%    
Revolving Credit Facility | Credit Agreement | Minimum | Prime Commercial Lending Rate      
Line Of Credit Facility      
Line of credit facility interest rate 0.125%    
Revolving Credit Facility | Credit Agreement | Minimum | Libor Rate      
Line Of Credit Facility      
Line of credit facility interest rate 1.125%    
Revolving Credit Facility | Credit Agreement | Maximum | Prime Commercial Lending Rate      
Line Of Credit Facility      
Line of credit facility interest rate 0.75%    
Revolving Credit Facility | Credit Agreement | Maximum | Libor Rate      
Line Of Credit Facility      
Line of credit facility interest rate 1.75%    
Revolving Credit Facility | Credit Agreement | Letter of Credit      
Line Of Credit Facility      
Revolving credit facility, maximum borrowing capacity     $ 25,000,000