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Organization and Basis of Presentation - Summary of Financial Securities (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017
Held to maturity |  |
Equity and Debt Securities, Amortized Cost | $ 195,850 | $ 187,155
Equity and Debt Securities, Gross Unrealized Gain (Loss) | 1,895 | 2,849
Equity and Debt Securities, Marketable Securities | 197,745 | 190,004
Investment Funds - Debt Securities [Member] |  |
Equity securities: |  |
Amortized Cost | 26,946 | 27,914
Gross Unrealized Gain (Loss) | (360) | (29)
Carrying Value | 26,586 | 27,885
Investment Funds - U.S. Treasuries [Member] |  |
Equity securities: |  |
Amortized Cost | 11,250 | 10,999
Gross Unrealized Gain (Loss) | (86) | (55)
Carrying Value | 11,164 | 10,944
Common Stock and Stock Funds [Member] |  |
Equity securities: |  |
Amortized Cost | 39,620 | 34,329
Gross Unrealized Gain (Loss) | 2,459 | 2,973
Carrying Value | 42,079 | 37,302
Investment-Grade Unsecured Bonds [Member] |  |
Available for sale |  |
Amortized Cost | 4,324 | 4,365
Gross Unrealized Gain (Loss) | (118) | (40)
Carrying Value | 4,206 | 4,325
Mortgage Backed Securities [Member] |  |
Held to maturity |  |
Amortized Cost | 113,710 | 109,548
Gross Unrealized Gain (Loss) | 0 | 0
Carrying Value | $ 113,710 | $ 109,548

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