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Marketable Securities (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Marketable Securities [Abstract]        
Available-for-sale Securities, Gross Unrealized Gain     $ 37  
Available-for-sale Securities, Gross Realized Gain (Loss) $ 24 $ (3) 52 $ 776
Investment Income, Amortization of Premium $ 219 $ 307 $ 771 $ 925