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Marketable Securities (Detail) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Marketable Securities [Abstract]        
Available-for-sale Securities, Gross Realized Gains $ 5,000 $ 49,000 $ 5 $ 182
Available-for-sale Securities, Gross Unrealized Loss       114,000
Investment Income, Amortization of Premium $ 88,000 $ 183,000 $ 218,000 $ 382,000