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Investments in Debt and Equity Securities - Additional Information (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]    
Available for sale investments gross pre-tax unrealized gain (loss) $ 8,416 $ 3,701
Proceeds from sale of investments available for sale 6,408 14,971
Pre tax reclassification of net investment realized gain into net income 360 690
Reclassification of net investment realized gain into net income, net of tax 220 423
Certificates of deposit, commercial paper and money market funds due within one year    
Schedule of Trading Securities and Other Trading Assets [Line Items]    
Unrealized gains on investments, pre tax $ 5,075 $ 1,802