Investments - Other Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investment [Line Items]
Cost method	$ 8,039	$ 7,629
Other	574	564
Total other investments	8,613	8,193
CoBank
Investment [Line Items]
Cost method	7,258	6,818
Other Equity Investments
Investment [Line Items]
Cost method	781	811
Other	$ 574	$ 564