Accounting For Income Taxes (Schedule Of Deferred Income Tax Assets And Liabilities) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Valuation Allowance [Line Items]
Unfunded benefit obligation	$ 43,224	$ 45,842
Utility energy commodity and interest rate swap derivatives	8,436	11,724
Regulatory deferred tax credits	6,394	6,244
Tax credits	21,696	21,008
Power and natural gas deferrals	8,624	17,618
Deferred compensation	7,171	5,536
Deferred taxes on regulatory liabilities	101,648	106,909
Other	17,423	16,793
Total gross deferred income tax assets	214,616	231,674
Valuation allowances for deferred tax assets	(16,550)	(13,651)
Total gross deferred income tax assets	198,066	218,023
Differences between book and tax basis of utility plant	525,931	509,789
Regulatory asset on utility, property plant and equipment	86,701	83,141
Regulatory asset for pensions and other postretirement benefits	43,838	47,893
Utility energy commodity and interest rate swap derivatives	8,436	11,724
Long-term debt and borrowing costs	26,552	24,609
Settlement with Coeur d’Alene Tribe	6,250	6,400
Other regulatory assets	20,137	15,318
Other	8,734	6,751
Total deferred income tax liabilities	726,579	705,625
Total deferred income tax liabilities	528,513	$ 487,602
State Tax Credit Carryforward [Member]
Valuation Allowance [Line Items]
Tax Credit Carryforward, Amount	22,300
Tax Credit Carryforwards, Net of Valuation Allowance	6,000
Tax Credit Carryforward, Valuation Allowance	$ (16,300)
Minimum [Member] | State Tax Credit Carryforward [Member]
Valuation Allowance [Line Items]
Tax Credit Carryforward, Expiration Date	2020
Maximum [Member] | State Tax Credit Carryforward [Member]
Valuation Allowance [Line Items]
Tax Credit Carryforward, Expiration Date	2033