REGULATORY ASSETS & REGULATORY LIABILITIES (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 326,406	$ 352,656
Environmental Remediation Costs: Expended - Net [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	30,387	37,892
Environmental Restoration Costs: Liability for Future Expenditures [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	110,148	107,410
Deferred Asset Retirement Obligation Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	30,594	30,199
Deferred Pension and Other Postretirement Benefit Costs [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	95,897	95,897
Conservation Incentive Program Receivable [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	11,824	31,686
Societal Benefit Costs Receivable [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	12,502	12,801
Premium for Early Retirement of Debt [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	997	1,075
Deferred Interest Rate Contracts [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	5,164	7,761
Energy Efficiency Tracker [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	11,949	12,306
Pipeline Supplier Service Charges [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	7,938	8,771
Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 9,006	$ 6,858