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Fair Value of Financial Instruments (Available-for-sale Securities) (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Fair Value of Financial Instruments [Abstract]    
Amortized cost $ 348,690 $ 324,875
Unrealized gains (losses) on available-for-sale securities (551) (493)
Fair value $ 348,139 $ 324,382