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Fair Value of Financial Instruments (Unrealized gain loss on trading securities) (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Fair Value of Financial Instruments [Abstract]        
Unrealized gains (losses) on trading securities held in rabbi trust $ 359 $ 183 $ 822 $ 286