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Revolving Line of Credit - Additional Information (Detail)
12 Months Ended
Nov. 16, 2015
USD ($)
Dec. 31, 2018
USD ($)
letters_of_credit
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Line Of Credit Facility [Line Items]        
Outstanding borrowings   $ 0 $ 0  
Number of outstanding letter of credit | letters_of_credit   0    
Credit Agreement        
Line Of Credit Facility [Line Items]        
Interest paid   $ 770,000 $ 804,000 $ 1,900,000
Revolving Credit Facility | Credit Agreement        
Line Of Credit Facility [Line Items]        
Revolving credit facility, maximum borrowing capacity $ 300,000,000      
Revolving line of credit maturity date Nov. 16, 2020      
Debt instrument, interest rate, stated percentage   5.75%    
Outstanding borrowings   $ 0    
Line of credit facility, unused borrowing capacity   $ 300,000,000    
Revolving Credit Facility | Credit Agreement | LIBOR Rate        
Line Of Credit Facility [Line Items]        
Debt instrument, interest rate, effective percentage   3.77%    
Revolving Credit Facility | Credit Agreement | Minimum | Prime Commercial Lending Rate        
Line Of Credit Facility [Line Items]        
Line of credit facility interest rate 0.25%      
Revolving Credit Facility | Credit Agreement | Minimum | LIBOR Rate        
Line Of Credit Facility [Line Items]        
Line of credit facility interest rate 1.25%      
Revolving Credit Facility | Credit Agreement | Maximum | Prime Commercial Lending Rate        
Line Of Credit Facility [Line Items]        
Line of credit facility interest rate 1.00%      
Revolving Credit Facility | Credit Agreement | Maximum | LIBOR Rate        
Line Of Credit Facility [Line Items]        
Line of credit facility interest rate 2.00%      
Letter of Credit | Revolving Credit Facility | Credit Agreement        
Line Of Credit Facility [Line Items]        
Revolving credit facility, maximum borrowing capacity $ 10,000,000