INVESTMENTS IN OTHER ASSOCIATIONS (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Investments in other associations
Investment in other associations	$ 144,632		$ 143,608
Impairment during the period	0	$ 0
Basin Electric Power Cooperative
Investments in other associations
Investment in other associations	101,820		101,820
National Rural Utilities Cooperative Finance Corporation - patronage capital
Investments in other associations
Investment in other associations	11,232		11,232
National Rural Utilities Cooperative Finance Corporation - capital term certificates
Investments in other associations
Investment in other associations	16,021		16,085
CoBank, ACB
Investments in other associations
Investment in other associations	9,062		8,174
Western Fuels Association
Investments in other associations
Investment in other associations	2,336		2,346
Other
Investments in other associations
Investment in other associations	$ 4,161		$ 3,951