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Revolving Line of Credit - Additional Information (Detail) - Revolving Credit Facility - Credit Agreement - USD ($)
6 Months Ended
Jun. 30, 2018
Nov. 16, 2015
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 $ 300,000,000  
Prime Commercial Lending Rate    
Line Of Credit Facility [Line Items]    
Effective percentage interest rate 5.25%  
Libor Rate    
Line Of Credit Facility [Line Items]    
Effective percentage interest rate 3.34%  
Minimum | Prime Commercial Lending Rate    
Line Of Credit Facility [Line Items]    
Line of credit facility interest rate 0.25%  
Minimum | Libor Rate    
Line Of Credit Facility [Line Items]    
Line of credit facility interest rate 1.25%  
Maximum | Prime Commercial Lending Rate    
Line Of Credit Facility [Line Items]    
Line of credit facility interest rate 1.00%  
Maximum | Libor Rate    
Line Of Credit Facility [Line Items]    
Line of credit facility interest rate 2.00%  
Letter of Credit    
Line Of Credit Facility [Line Items]    
Revolving credit facility, maximum borrowing capacity   $ 10,000,000