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Investments in Marketable Securities Available-for-Sale (Details 1) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]      
Proceeds from sales $ 47,119 $ 38,532 $ 46,720
Gross realized gains 204 137 261
Gross realized losses $ (16) $ (127) $ (38)