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Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Thousands | 6 Months Ended |
--- | --- | ---
Jun. 30, 2018 | Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Securities, carrying amount | $ 112,452 | $ 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

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