Accounting For Income Taxes (Schedule Of Deferred Income Tax Assets And Liabilities) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Valuation Allowance [Line Items]
Unfunded benefit obligation	$ 45,842	$ 41,944
Utility energy commodity and interest rate swap derivatives	11,724	23,364
Regulatory deferred tax credits	6,244	6,359
Tax credits	21,008	23,042
Power and natural gas deferrals	17,618	14,379
Deferred compensation	5,536	7,080
Deferred taxes on regulatory liabilities	106,909	105,508
Other	16,793	15,892
Total gross deferred income tax assets	231,674	237,568
Valuation allowances for deferred tax assets	(13,651)	(10,982)
Total gross deferred income tax assets	218,023	226,586
Differences between book and tax basis of utility plant	509,789	494,783
Regulatory asset on utility, property plant and equipment	83,141	81,860
Regulatory asset for pensions and other postretirement benefits	47,893	43,914
Utility energy commodity and interest rate swap derivatives	11,724	23,364
Long-term debt and borrowing costs	24,609	19,992
Settlement with Coeur d’Alene Tribe	6,400	6,802
Other regulatory assets	15,318	16,695
Other	6,751	5,806
Total deferred income tax liabilities	705,625	693,216
Total deferred income tax liabilities	487,602	$ 466,630
State Tax Credit Carryforward [Member]
Valuation Allowance [Line Items]
Tax Credit Carryforward, Amount	21,000
Tax Credit Carryforwards, Net of Valuation Allowance	7,300
Tax Credit Carryforward, Valuation Allowance	$ (13,700)
Minimum [Member] | State Tax Credit Carryforward [Member]
Valuation Allowance [Line Items]
Tax Credit Carryforward, Expiration Date	Dec. 31, 2020
Maximum [Member] | State Tax Credit Carryforward [Member]
Valuation Allowance [Line Items]
Tax Credit Carryforward, Expiration Date	Dec. 31, 2032