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Investments - Schedule of Investments (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]    
Short-Term $ 10,292 $ 11,286
Long-Term 80,682 78,975
Municipal Debt Securities [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Short-Term 8,312 10,306
Long-Term 46,195 46,513
Federal Home Loan Securities [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Short-Term 1,000  
Long-Term 30,322 28,747
Certificates of Deposit [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Short-Term 980 980
Long-Term $ 4,165 $ 3,715