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Investments (Narrative) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Investments [Abstract]      
Investment maturity term 1 year 1 year  
Realized gains (losses) on available-for-sale securities $ 0 $ 547 $ 0
Other-than-temporary impairment of available-for-sale securities     $ 244