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Investments Narrative (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule of Investments [Line Items] |  |
Investments—affiliates | $ 1,579 | $ 130,785
Trading securities | 51,700 | 65,500
Domestic and Foreign Debt Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Trading securities | 42,200 | 45,500
Equity Securities [Member] |  |
Schedule of Investments [Line Items] |  |
Trading securities | 4,500 | 7,200
Equity Securities - Small and Mid-Cap [Member] |  |
Schedule of Investments [Line Items] |  |
Trading securities | 4,400 | 2,800
Mutual Fund [Member] |  |
Schedule of Investments [Line Items] |  |
Trading securities | $ 600 | $ 8,900

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