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Fair Value of Financial Instruments (Narrative) (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Fair Value of Financial Instruments [Abstract]          
Investment maturity term     1 year    
Realized gains (losses) on available-for-sale securities $ 0 $ 0 $ 0 $ 547  
Fair value of investments in rabbi trust $ 19,157   $ 19,157   $ 17,843