Regulatory Matters (Schedule Of Asset And Liability) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Regulated Asset Liability [Line Items]
Earning a return, asset	$ 190,643
Not earning a return, asset	384,065
Pending regulatory treatment, asset	4,924
Total, asset	579,632	$ 579,000
Earning a return, liability	302,989
Not earning a return, liability	27,471
Pending Regulatory Treatment Liability	3,423
Total, liability	333,883	333,628
Natural Gas Deferrals [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	17,880
Not earning a return, liability	0
Pending Regulatory Treatment Liability	0
Total, liability	17,880	3,921
Power Deferrals [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	18,747
Not earning a return, liability	0
Pending Regulatory Treatment Liability	0
Total, liability	18,747	14,186
Removal Costs [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	261,594
Not earning a return, liability	0
Pending Regulatory Treatment Liability	0
Total, liability	261,594	254,140
Income Tax Related Liabilities [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	0
Not earning a return, liability	17,609
Pending Regulatory Treatment Liability	0
Total, liability	17,609	14,534
Regulatory Liability For Production Facility [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	0
Not earning a return, liability	0
Pending Regulatory Treatment Liability	0
Total, liability	0	29,028
Other Regulatory Assets [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	2,395
Not earning a return, liability	1,048
Pending Regulatory Treatment Liability	0
Total, liability	3,443	7,688
Revenue Subject to Refund [Member]
Regulated Asset Liability [Line Items]
Earning a return, liability	0
Not earning a return, liability	8,814
Pending Regulatory Treatment Liability	3,423
Total, liability	$ 12,237	10,131
Decoupling [Member]
Regulated Asset Liability [Line Items]
Remaining amortization period, regulatory liability	2 years
Earning a return, liability	$ 2,373
Not earning a return, liability	0
Pending Regulatory Treatment Liability	0
Total, liability	$ 2,373	0
Investment In Exchange Power-Net [Member]
Regulated Asset Liability [Line Items]
Remaining amortization period, regulatory assets	4 years
Earning a return, asset	$ 8,983
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Total, asset	8,983	11,433
Deferred Income Tax Charge [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	101,240
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Total, asset	101,240	100,412
Pension and Other Postretirement Plans Costs [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	0
Not earning a return, asset	235,009
Pending regulatory treatment, asset	0
Total, asset	235,009	235,758
Current Regulatory Asset For Utility Derivatives [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	0
Not earning a return, asset	17,260
Pending regulatory treatment, asset	0
Total, asset	17,260	29,640
Unamortized Debt Repurchase Costs [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	15,520
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Total, asset	$ 15,520	17,357
Regulatory Asset For Settlement With Coeur d'Alene Tribe [Member]
Regulated Asset Liability [Line Items]
Remaining amortization period, regulatory assets	44 years
Earning a return, asset	$ 46,576
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Total, asset	46,576	47,887
Demand Side Management Programs [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	0
Not earning a return, asset	3,168
Pending regulatory treatment, asset	0
Total, asset	3,168	4,603
Montana Lease Payments [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	947
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Total, asset	$ 947	1,984
Lancaster Plant 2010 Net Costs [Member]
Regulated Asset Liability [Line Items]
Remaining amortization period, regulatory assets	0 years
Earning a return, asset	$ 0
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Total, asset	$ 0	1,247
Deferred Maintenance Costs [Member]
Regulated Asset Liability [Line Items]
Remaining amortization period, regulatory assets	2 years
Earning a return, asset	$ 0
Not earning a return, asset	4,823
Pending regulatory treatment, asset	0
Total, asset	$ 4,823	5,804
Decoupling [Member]
Regulated Asset Liability [Line Items]
Remaining amortization period, regulatory assets	2 years
Earning a return, asset	$ 13,312
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Total, asset	13,312	0
Power Deferrals Regulatory Asset [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	933
Not earning a return, asset	0
Pending regulatory treatment, asset	0
Total, asset	933	8,291
Regulatory Asset For Interest Rate Swaps [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	0
Not earning a return, asset	83,973
Pending regulatory treatment, asset	0
Total, asset	83,973	77,063
Non Current Regulatory Asset For Utility Derivatives [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	0
Not earning a return, asset	32,420
Pending regulatory treatment, asset	0
Total, asset	32,420	24,483
Other Regulatory Assets [Member]
Regulated Asset Liability [Line Items]
Earning a return, asset	3,132
Not earning a return, asset	7,412
Pending regulatory treatment, asset	4,924
Total, asset	$ 15,468	$ 13,038