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Accounting For Income Taxes (Schedule Of Deferred Income Tax Assets And Liabilities) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Valuation Allowance [Line Items]    
Unfunded benefit obligation $ 41,944 $ 80,230
Utility energy commodity and interest rate swap derivatives 23,364 31,872
Regulatory deferred tax credits 6,359 15,192
Tax credits 23,042 27,931
Power and natural gas deferrals 14,379 19,415
Deferred compensation 7,080 11,141
Deferred taxes on regulatory liabilities 105,508 6,604
Other 15,892 22,908
Total gross deferred income tax assets 237,568 215,293
Valuation allowances for deferred tax assets (10,982) (7,946)
Total gross deferred income tax assets 226,586 207,347
Differences between book and tax basis of utility plant 494,783 812,916
Regulatory asset on utility, property plant and equipment 81,860 37,301
Regulatory asset for pensions and other postretirement benefits 43,914 84,040
Utility energy commodity and interest rate swap derivatives 23,364 31,871
Long-term debt and borrowing costs 19,992 31,955
Settlement with Coeur d’Alene Tribe 6,802 11,711
Other regulatory assets 16,695 30,183
Other 5,806 8,298
Total deferred income tax liabilities 693,216 1,048,275
Total deferred income tax liabilities 466,630 $ 840,928
State Tax Credit Carryforward [Member]    
Valuation Allowance [Line Items]    
Tax Credit Carryforward, Amount 19,600  
Tax Credit Carryforwards, Net of Valuation Allowance 8,600  
Tax Credit Carryforward, Valuation Allowance (11,000)  
Federal Tax Credit Carryforward [Member]    
Valuation Allowance [Line Items]    
Tax Credit Carryforward, Amount $ 3,500  
Tax Credit Carryforward, Expiration Date Dec. 31, 2036  
Minimum [Member] | State Tax Credit Carryforward [Member]    
Valuation Allowance [Line Items]    
Tax Credit Carryforward, Expiration Date Dec. 31, 2019  
Maximum [Member] | State Tax Credit Carryforward [Member]    
Valuation Allowance [Line Items]    
Tax Credit Carryforward, Expiration Date Dec. 31, 2028