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Marketable Securities (Detail) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Marketable Securities [Abstract]        
Available-for-sale Securities, Gross Unrealized Gain     $ 873,000 $ 277,000
Available-for-sale Securities, Gross Realized Gain (Loss) $ 0 $ 0 779,000 (96,000)
Investment Income, Amortization of Premium $ 309,000 $ 348,000 $ 617,000 $ 693,000