Investments - Other Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investment [Line Items]
Total investments carried at fair value	$ 2,037	$ 1,779
Total investments carried at cost	8,704	8,487
Total equity method investments	533	522
Total investments	11,274	10,788
CoBank
Investment [Line Items]
Total investments carried at cost	7,925	7,705
Other
Investment [Line Items]
Total investments carried at cost	779	782
Total equity method investments	533	522
Domestic equity funds
Investment [Line Items]
Total investments carried at fair value	1,628	1,409
International equity funds
Investment [Line Items]
Total investments carried at fair value	$ 409	$ 370