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Organization and Basis of Presentation - Summary of Financial Securities (Details) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Dec. 31, 2017
Held to maturity |  |
Equity and Debt Securities, Amortized Cost | $ 207,402 | $ 187,155
Equity and Debt Securities, Gross Unrealized Gain (Loss) | 3,194 | 2,849
Equity and Debt Securities, Marketable Securities | 210,596 | 190,004
Investment funds - debt securities [Member] |  |
Equity securities: |  |
Amortized Cost | 31,914 | 27,914
Gross Unrealized Gain (Loss) | (430) | (29)
Carrying Value | 31,484 | 27,885
Investment funds - U.S. treasuries [Member] |  |
Equity securities: |  |
Amortized Cost | 8,929 | 10,999
Gross Unrealized Gain (Loss) | (90) | (55)
Carrying Value | 8,839 | 10,944
Common stock and stock funds [Member] |  |
Equity securities: |  |
Amortized Cost | 39,755 | 34,329
Gross Unrealized Gain (Loss) | 3,835 | 2,973
Carrying Value | 43,590 | 37,302
Investment-grade unsecured bonds [Member] |  |
Available for sale |  |
Amortized Cost | 4,281 | 4,365
Gross Unrealized Gain (Loss) | (121) | (40)
Carrying Value | 4,160 | 4,325
Mortgage backed securities [Member] |  |
Held to maturity |  |
Amortized Cost | 122,523 | 109,548
Gross Unrealized Gain (Loss) | 0 | 0
Carrying Value | $ 122,523 | $ 109,548

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