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Marketable Securities (Detail) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Marketable Securities [Abstract]        
Available-for-sale Securities, Gross Realized Gains     $ 5,000 $ 182,000
Available-for-sale Securities, Gross Unrealized Loss       113,000
Investment Income, Amortization of Premium $ 0 $ 175,000 $ 218,000 $ 557,000