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Summary of Significant Accounting Policies - Narrative (Details) - USD ($)
$ in Thousands
 9 Months Ended
Sep. 30, 2018
Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Securities, carrying amount $ 103,921 $ 93,383
Operating Leases, Future Minimum Payments Due, Percent Of Current Assets 1.00%  
Equity Funds [Member]    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Securities, carrying amount $ 0 $ 499