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Marketable Securities (Detail) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Marketable Securities [Abstract]        
Available-for-sale Securities, Gross Unrealized Gain     $ 939,000  
Available-for-sale Securities, Gross Realized Gain (Loss) $ 3,000 $ (14,000) 776,000 $ (82,000)
Investment Income, Amortization of Premium $ 307,000 $ 347,000 $ 925,000 $ 1,040,000