Jointly owned facilities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Big Stone Station:
Jointly owned facilities [Line Items]
Utility plant in service	$ 157,144	$ 157,761
Less accumulated depreciation	49,568	48,242
Utility plant in services net	107,576	109,519
Coyote Station:
Jointly owned facilities [Line Items]
Utility plant in service	156,334	140,895
Less accumulated depreciation	105,928	94,755
Utility plant in services net	50,406	46,140
Wygen III:
Jointly owned facilities [Line Items]
Utility plant in service	66,251	65,023
Less accumulated depreciation	7,550	6,788
Utility plant in services net	$ 58,701	$ 58,235