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Fair Value of Financial Instruments (Available-for-sale securities) (Details) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Fair Value of Financial Instruments [Abstract]    
Amortized cost $ 435,193 $ 455,109
Unrealized gains (losses) on available-for-sale securities (316) (218)
Fair market value $ 434,877 $ 454,891