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Revolving Line of Credit - Additional Information (Detail)
6 Months Ended
Jun. 30, 2017
USD ($)
letters_of_credit
Dec. 31, 2016
USD ($)
Nov. 16, 2015
USD ($)
Line Of Credit Facility [Line Items]      
Outstanding borrowings $ 0 $ 10,000,000  
Number of letters of credit | letters_of_credit 2    
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, description of variable rate basis Borrowings under the Credit Facility bear interest at a rate of either (1) Wells Fargo Bank’s prime rate (subject to certain higher rate determinations) plus a margin of 0.25% to 1.00% or (2) the 30, 60, 90 or 180 day LIBOR rate plus a margin of 1.25% to 2.00%.    
Outstanding borrowings $ 0    
Line of credit facility, unused borrowing capacity $ 297,800,000    
Revolving Credit Facility | Credit Agreement | Prime Commercial Lending Rate      
Line Of Credit Facility [Line Items]      
Effective percentage interest rate 4.50%    
Revolving Credit Facility | Credit Agreement | Libor Rate      
Line Of Credit Facility [Line Items]      
Effective percentage interest rate 2.37%    
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 $ 2,200,000    
Letter of Credit | Revolving Credit Facility | Credit Agreement      
Line Of Credit Facility [Line Items]      
Revolving credit facility, maximum borrowing capacity     $ 10,000,000