Other Comprehensive Income (Loss): Reclassification Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Condensed Statement of Income Captions [Line Items]
Interest Expense	$ 35,765		$ 34,660		$ 71,546	$ 69,676
Fuel, purchased power and cost of natural gas sold	71,629		90,200		259,508	339,942
Operations and maintenance	117,308		124,950		242,774	248,534
Income before Income Taxes	29,525		20,000		142,751	144,625
Income tax benefit (expense)	(4,831)		(2,307)		(20,833)	(19,570)
Net income (loss)	24,694	$ 97,224	17,693	$ 107,362	121,918	125,055
Reclassification out of Accumulated Other Comprehensive Income
Condensed Statement of Income Captions [Line Items]
Net income (loss)	(993)		(631)		(2,386)	(908)
Cash Flow Hedging | Reclassification out of Accumulated Other Comprehensive Income
Condensed Statement of Income Captions [Line Items]
Income before Income Taxes	(783)		(630)		(1,982)	(789)
Income tax benefit (expense)	186		153		471	188
Net income (loss)	(597)		(477)		(1,511)	(601)
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	25		20		55	39
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)		(221)		(1,194)	(441)
Accumulated Defined Benefit Plans Adjustment | Reclassification out of Accumulated Other Comprehensive Income
Condensed Statement of Income Captions [Line Items]
Income before Income Taxes	(572)		(201)		(1,139)	(402)
Income tax benefit (expense)	176		47		264	95
Net income (loss)	(396)		(154)		(875)	(307)
Interest rate swaps | Cash Flow Hedging | Reclassification out of Accumulated Other Comprehensive Income
Condensed Statement of Income Captions [Line Items]
Interest Expense	(713)		(713)		(1,426)	(1,426)
Commodity Contract | 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	$ (70)		$ 83		$ (556)	$ 637