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Investments - Summary of Investments (Detail) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]    
Adjusted Cost $ 76,783 $ 89,683
Gross Unrealized Gains 2  
Gross Unrealized (Losses) (263) (412)
Estimated Fair Value 76,522 89,271
Certificates of Deposit [Member]    
Debt Securities, Available-for-sale [Line Items]    
Adjusted Cost   4,915
Gross Unrealized (Losses)   (3)
Estimated Fair Value   4,912
Municipal Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Adjusted Cost 76,783 84,768
Gross Unrealized Gains 2  
Gross Unrealized (Losses) (263) (409)
Estimated Fair Value $ 76,522 $ 84,359