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Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Statement [Abstract]      
Unrealized gain( loss) on investments, tax $ 132 $ 0 $ (5)
Reclassification of unrealized (gain) loss on investments into earnings, tax $ (91) $ 24 $ 0