REGULATORY ASSETS AND LIABILITIES (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Regulatory Assets [Line Items]
Total regulatory assets	$ 324,161	$ 315,221
Environmental Restoration Costs: Expended - Net [Member]
Regulatory Assets [Line Items]
Total regulatory assets	40,773	45,815
Environmental Restoration Costs: Liability for Future Expenditures [Member]
Regulatory Assets [Line Items]
Total regulatory assets	91,512	89,984
Deferred Asset Retirement Obligation Costs [Member]
Regulatory Assets [Line Items]
Total regulatory assets	25,477	25,162
Deferred Pension and Other Postretirement Benefit Costs [Member]
Regulatory Assets [Line Items]
Total regulatory assets	88,340	88,624
Deferred Gas Costs - Net [Member]
Regulatory Assets [Line Items]
Total regulatory assets	4,638	22,441
Conservation Incentive Program Receivable [Member]
Regulatory Assets [Line Items]
Total regulatory assets	33,924	13,580
Societal Benefit Costs Receivable [Member]
Regulatory Assets [Line Items]
Total regulatory assets	11,827	8,618
Premium for Early Retirement of Debt [Member]
Regulatory Assets [Line Items]
Total regulatory assets	416	537
Deferred Interest Rate Contracts [Member]
Regulatory Assets [Line Items]
Total regulatory assets	8,365	8,146
Energy Efficiency Tracker [Member]
Regulatory Assets [Line Items]
Total regulatory assets	12,974	8,464
Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total regulatory assets	$ 5,915	$ 3,850