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Revolving Line of Credit - Additional Information (Detail)
12 Months Ended
Dec. 31, 2017
USD ($)
letters_of_credit
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Nov. 16, 2015
USD ($)
Line Of Credit Facility [Line Items]        
Outstanding borrowings $ 0 $ 10,000,000    
Number of outstanding letter of credit | letters_of_credit 1      
Credit Agreement        
Line Of Credit Facility [Line Items]        
Interest paid $ 804,000 $ 1,900,000 $ 223,000  
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 4.75%      
Outstanding borrowings $ 0      
Line of credit facility, unused borrowing capacity $ 299,500,000      
Revolving Credit Facility | Credit Agreement | LIBOR Rate        
Line Of Credit Facility [Line Items]        
Debt instrument, interest rate, effective percentage 2.78%      
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 | Credit Agreement        
Line Of Credit Facility [Line Items]        
Letters of credit, outstanding $ 500,000      
Line of credit maturity period 2018      
Letter of Credit | Revolving Credit Facility | Credit Agreement        
Line Of Credit Facility [Line Items]        
Revolving credit facility, maximum borrowing capacity       $ 10,000,000