Investments - Other Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Investment [Line Items]
Cost method:	$ 8,225	$ 7,629
Equity method:	575	564
Total other investments	8,800	8,193
CoBank
Investment [Line Items]
Cost method:	7,441	6,818
Other
Investment [Line Items]
Cost method:	784	811
Equity method:	$ 575	$ 564