Other Comprehensive Income (Loss): Reclassification Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Condensed Statement of Income Captions [Line Items]
Interest Expense	$ 36,521			$ 34,000			$ 108,067	$ 103,677
Fuel, purchased power and cost of natural gas sold	71,686			73,544			331,194	413,486
Income before Income Taxes	45,000			17,903			187,751	162,528
Income tax benefit (expense)	(4,651)			(2,508)			(25,484)	(22,078)
Net income (loss)	40,349	$ 24,694	$ 97,224	15,395	$ 17,693	$ 107,362	162,267	140,450
Operations and maintenance	122,759			116,583			365,533	365,116
Reclassification out of Accumulated Other Comprehensive Income
Condensed Statement of Income Captions [Line Items]
Net income (loss)	(1,111)			(647)			(3,497)	(1,555)
Cash Flow Hedging | Reclassification out of Accumulated Other Comprehensive Income
Condensed Statement of Income Captions [Line Items]
Income before Income Taxes	(890)			(842)			(2,872)	(1,631)
Income tax benefit (expense)	209			170			680	358
Net income (loss)	(681)			(672)			(2,192)	(1,273)
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost | Reclassification out of Accumulated Other Comprehensive Income
Condensed Statement of Income Captions [Line Items]
Operations and maintenance	24			19			79	58
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) | Reclassification out of Accumulated Other Comprehensive Income
Condensed Statement of Income Captions [Line Items]
Operations and maintenance	(597)			(83)			(1,791)	(524)
Accumulated Defined Benefit Plans Adjustment | Reclassification out of Accumulated Other Comprehensive Income
Condensed Statement of Income Captions [Line Items]
Income before Income Taxes	(573)			(64)			(1,712)	(466)
Income tax benefit (expense)	143			89			407	184
Net income (loss)	(430)			25			(1,305)	(282)
Interest rate swaps | Cash Flow Hedging | Reclassification out of Accumulated Other Comprehensive Income
Condensed Statement of Income Captions [Line Items]
Interest Expense	(712)			(713)			(2,138)	(2,139)
Commodity Contracts | Cash Flow Hedging | Reclassification out of Accumulated Other Comprehensive Income
Condensed Statement of Income Captions [Line Items]
Fuel, purchased power and cost of natural gas sold	$ (178)			$ (129)			$ (734)	$ 508