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Marketable Securities (Detail) - USD ($)
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Marketable Securities [Abstract]    
Available-for-sale Securities, Gross Unrealized Gain $ 296,000 $ 2,000
Available-for-sale Securities, Gross Realized Gain (Loss) 779,000 (96,000)
Investment Income, Amortization of Premium $ 308,000 $ 345,000