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Investments - Narrative (Details)
12 Months Ended
Dec. 31, 2017
USD ($)
investment
Dec. 31, 2016
USD ($)
Investments, Debt and Equity Securities [Abstract]    
Investment impairment charges $ 215,000 $ 139,000
Other than temporary impairments $ 0 $ 0
Number of securities in a gross unrealized loss position | investment 42  
Number of securities in a gross unrealized loss position for more than twelve months | investment 16  
Unrealized loss $ 108,000