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Marketable Securities (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Marketable Securities [Abstract]    
Available-for-sale Securities, Gross Realized Gains   $ (26)
Available-for-sale Securities, Gross Realized Losses $ (133) (6)
Available-for-sale Securities, Gross Unrealized Gain   1
Available-for-sale Securities, Gross Unrealized Gain (Loss) 82  
Available-for-sale Securities, Gross Unrealized Loss 118 208
Investment Income, Amortization of Premium $ 199 $ 308