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Investments (Available-for-sale Securities) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Investments [Abstract]    
Amortized cost $ 324,875 $ 455,109
Unrealized gains (losses) on available-for-sale securities (493) (218)
Fair value $ 324,382 $ 454,891