REGULATORY ASSETS & REGULATORY LIABILITIES (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Regulatory Assets [Line Items]
Total regulatory assets	$ 257,632	$ 248,413
Environmental Remediation Costs: Expended - Net [Member]
Regulatory Assets [Line Items]
Total regulatory assets	48,199	39,056
Environmental Restoration Costs: Liability for Future Expenditures [Member]
Regulatory Assets [Line Items]
Total regulatory assets	81,357	87,393
Income Taxes - Flowthrough Depreciation [Member]
Regulatory Assets [Line Items]
Total regulatory assets	40	774
Deferred Asset Retirement Obligation Costs [Member]
Regulatory Assets [Line Items]
Total regulatory assets	24,781	24,247
Pension Costs [Member]
Regulatory Assets [Line Items]
Total regulatory assets	68,733	69,017
Deferred Fuel Costs [Member]
Regulatory Assets [Line Items]
Total regulatory assets	4,750	0
Conservation Incentive Program Recevable [Member]
Regulatory Assets [Line Items]
Total regulatory assets	4,805	12,291
Societal Benefit Costs Receivable [Member]
Regulatory Assets [Line Items]
Total regulatory assets	6,919	4,216
Premium for Early Retirement of debt [Member]
Regulatory Assets [Line Items]
Total regulatory assets	578	699
Deferred Interest Rate Contracts [Member]
Regulatory Assets [Line Items]
Total regulatory assets	7,825	3,150
Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total regulatory assets	$ 9,645	$ 7,570