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Marketable Securities (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Marketable Securities [Abstract]        
Available-for-sale Securities, Gross Unrealized Gain     $ 53  
Available-for-sale Securities, Gross Realized Gain (Loss) $ 9 $ 0 29 $ 779
Investment Income, Amortization of Premium $ 245 $ 309 $ 553 $ 617