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Revolving Line of Credit - Additional Information (Detail)
3 Months Ended
Mar. 31, 2018
USD ($)
letters_of_credit
Nov. 16, 2015
USD ($)
Line Of Credit Facility [Line Items]    
Number of letters of credit | letters_of_credit 1  
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 $ 299,500,000  
Revolving Credit Facility | Credit Agreement | Prime Commercial Lending Rate    
Line Of Credit Facility [Line Items]    
Effective percentage interest rate 5.00%  
Revolving Credit Facility | Credit Agreement | Libor Rate    
Line Of Credit Facility [Line Items]    
Effective percentage interest rate 2.50%  
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  
Letter of Credit | Revolving Credit Facility | Credit Agreement    
Line Of Credit Facility [Line Items]    
Revolving credit facility, maximum borrowing capacity   $ 10,000,000