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Line of Credit - Additional Information (Detail)
12 Months Ended
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Debt Instrument [Line Items]    
Borrowings under the line of credit agreement $ 0 $ 0
Letters of credit outstanding amount 3,100,000  
Available borrowing under letter of credit $ 96,900,000  
Maximum ratio of total liabilities to tangible net worth 2.50  
Company's tangible net worth ratio 0.001  
Line of Credit    
Debt Instrument [Line Items]    
Line of Credit Facility, Increase (Decrease), Net $ 100,000,000  
Line of Credit | Unsecured Revolving Line of Credit    
Debt Instrument [Line Items]    
Total availability under line of credit agreement 100,000,000  
Line of Credit | Letter of Credit    
Debt Instrument [Line Items]    
Total availability under line of credit agreement $ 10,000,000.0  
Minimum | Line of Credit | LIBOR    
Debt Instrument [Line Items]    
Debt instrument basis spread on variable rate 1.00%  
Maximum | Line of Credit | LIBOR    
Debt Instrument [Line Items]    
Debt instrument basis spread on variable rate 1.50%