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Marketable Securities (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Marketable Securities [Abstract]        
Available-for-sale Securities, Gross Realized Gains   $ 24   $ 52
Available-for-sale Securities, Gross Realized Losses     $ 182  
Available-for-sale Securities, Gross Unrealized Gain       4
Available-for-sale Securities, Gross Unrealized Gain (Loss)     113  
Available-for-sale Securities, Gross Unrealized Loss       93
Investment Income, Amortization of Premium $ 175 $ 219 $ 557 $ 771