Accounting For Income Taxes (Schedule Of Deferred Income Tax Assets And Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Valuation Allowance [Line Items]
Tax Credit Carryforwards, Net of Valuation Allowance	$ 9,200
Unfunded benefit obligation	80,230	$ 75,716
Derivatives	31,872	47,009
Deferred Tax Assets Regulatory Deferred Tax Credits	15,192	0
Tax credits	27,931	15,011
Power and natural gas deferrals	19,415	12,866
Deferred compensation	11,141	10,354
Other	29,512	29,471
Total gross deferred income tax assets	215,293	190,427
Valuation allowances for deferred tax assets	(7,946)	(2,862)
Total gross deferred income tax assets	207,347	187,565
Differences between book and tax basis of utility plant	812,916	723,661
Regulatory asset on utility, property plant and equipment	37,301	36,917
Regulatory asset for pensions and other postretirement benefits	84,040	82,253
Utility energy commodity derivatives	31,871	47,010
Long-term debt and borrowing costs	31,955	14,027
Settlement with Coeur d’Alene Tribe	11,711	12,084
Other regulatory assets	30,183	11,691
Other	8,298	7,399
Deferred Tax Liabilities, Gross	1,048,275	935,042
Total deferred income tax liabilities	840,928	$ 747,477
State Tax Credit Carryforward [Member]
Valuation Allowance [Line Items]
Tax Credit Carryforward, Valuation Allowance	$ (7,900)