Investments - Other Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Total investments carried at fair value	$ 1,779	$ 3,279
Total investments carried at cost	8,487	7,629
Total equity method investments	522	564
Total investments	10,788	11,472
CoBank
Investment [Line Items]
Total investments carried at cost	7,705	6,818
Other equity method investments
Investment [Line Items]
Total investments carried at cost	782	811
Total equity method investments	522	564
Domestic equity funds
Investment [Line Items]
Total investments carried at fair value	1,409	2,856
International equity funds
Investment [Line Items]
Total investments carried at fair value	$ 370	$ 423