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