Investments - Other Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Investment [Line Items]
Cost method investments	$ 6,919	$ 4,897
Equity method investments	450	3,128
Total other investments	7,369	8,025
Private Equity Limited Partnerships [Member]
Investment [Line Items]
Equity method investments	0	2,624
Other Equity Investments [Member]
Investment [Line Items]
Equity method investments	450	504
CoBank [Member]
Investment [Line Items]
Cost method investments	6,177	4,137
Other Equity Investments [Member]
Investment [Line Items]
Cost method investments	$ 742	$ 760