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Revolving Line of Credit (Detail) - USD ($)
9 Months Ended
Sep. 30, 2019
Dec. 31, 2018
Line Of Credit Facility [Line Items]    
Outstanding borrowings $ 0 $ 0
Revolving Credit Facility | Credit Agreement    
Line Of Credit Facility [Line Items]    
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 [Line Items]    
Effective percentage interest rate 5.25%  
Revolving Credit Facility | Credit Agreement | Libor Rate    
Line Of Credit Facility [Line Items]    
Effective percentage interest rate 3.14%  
Revolving Credit Facility | Credit Agreement | Minimum | Prime Commercial Lending Rate    
Line Of Credit Facility [Line Items]    
Line of credit facility interest rate 0.125%  
Revolving Credit Facility | Credit Agreement | Minimum | Libor Rate    
Line Of Credit Facility [Line Items]    
Line of credit facility interest rate 1.125%  
Revolving Credit Facility | Credit Agreement | Maximum | Prime Commercial Lending Rate    
Line Of Credit Facility [Line Items]    
Line of credit facility interest rate 0.75%  
Revolving Credit Facility | Credit Agreement | Maximum | Libor Rate    
Line Of Credit Facility [Line Items]    
Line of credit facility interest rate 1.75%  
Revolving Credit Facility | Credit Agreement | Letter of Credit    
Line Of Credit Facility [Line Items]    
Revolving credit facility, maximum borrowing capacity $ 25,000,000  
Secured Credit Facility | Credit Agreement    
Line Of Credit Facility [Line Items]    
Revolving credit facility, maximum borrowing capacity $ 300,000,000