Accounting For Income Taxes (Schedule Of Deferred Income Tax Assets And Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Valuation Allowance [Line Items]
Tax Credit Carryforwards, Net of Valuation Allowance	$ 12,400
Unfunded benefit obligation	75,716	$ 72,324
Derivatives	47,009	46,903
Tax credits	15,011	15,080
Power and natural gas deferrals	12,866	3,811
Deferred compensation	10,354	10,796
Other	29,471	20,583
Total gross deferred income tax assets	190,427	169,497
Valuation allowances for deferred tax assets	2,862	8,145
Total gross deferred income tax assets	187,565	161,352
Differences between book and tax basis of utility plant	723,661	654,321
Regulatory asset on utility, property plant and equipment	36,917	36,504
Regulatory asset for pensions and other postretirement benefits	82,253	82,515
Utility energy commodity derivatives	47,010	46,906
Long-term debt and borrowing costs	14,027	11,484
Settlement with Coeur d’Alene Tribe	12,084	12,458
Other regulatory assets	11,691	9,691
Other	7,399	3,021
Deferred Tax Liabilities, Gross	935,042	856,900
Total deferred income tax liabilities	747,477	695,548
Current deferred income tax asset (1)	0	14,794
Long-term deferred income tax liability (1)	747,477	$ 710,342
State Tax Credit Carryforward [Member]
Valuation Allowance [Line Items]
Tax Credit Carryforward, Valuation Allowance	$ 2,900