Jointly owned facilities (Tables)

12 Months Ended

Dec. 31, 2017

Regulated Operations [Abstract]

Company's share of the cost of utility plant in service and related accumulated depreciation for the stations

At December 31, the Company's share of the cost of utility plant in service and related accumulated depreciation for the stations was as follows:

	2017		2016
	(In thousands)
Big Stone Station:
Utility plant in service	$	158,084	$	157,144
Less accumulated depreciation	51,740		49,568
	$	106,344	$	107,576
Coyote Station:
Utility plant in service	$	155,287	$	156,334
Less accumulated depreciation	103,897		105,928
	$	51,390	$	50,406
Wygen III:
Utility plant in service	$	65,065	$	66,251
Less accumulated depreciation	7,652		7,550
	$	57,413	$	58,701