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Investments - Narrative (Details)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
USD ($)
investment
Sep. 30, 2017
USD ($)
Sep. 30, 2018
USD ($)
investment
Sep. 30, 2017
USD ($)
Investments, Debt and Equity Securities [Abstract]        
Investment impairment charges | $ $ 184 $ 11 $ 558 $ 78
Number of securities in gross unrealized loss position | investment 15   15  
Number of securities in gross unrealized loss position for longer than 12 months | investment 10   10  
Total unrealized loss | $ $ 167   $ 167