REGULATORY ASSETS & REGULATORY LIABILITIES (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 339,192	$ 357,160
Environmental Remediation Costs: Expended - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	32,797	29,540
Environmental Remediation Costs: Liability for Future Expenditures
Regulatory Assets [Line Items]
Total Regulatory Assets	125,466	124,308
Deferred Asset Retirement Obligation Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	31,976	31,584
Deferred Pension and Other Postretirement Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	99,040	99,040
Deferred Gas Costs - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	19,187	32,202
Societal Benefit Costs Receivable
Regulatory Assets [Line Items]
Total Regulatory Assets	0	385
Deferred Interest Rate Contracts
Regulatory Assets [Line Items]
Total Regulatory Assets	7,918	7,325
Energy Efficiency Tracker
Regulatory Assets [Line Items]
Total Regulatory Assets	545	11,247
Pipeline Supplier Service Charges
Regulatory Assets [Line Items]
Total Regulatory Assets	4,192	5,441
Pipeline Integrity Cost
Regulatory Assets [Line Items]
Total Regulatory Assets	4,544	3,431
AFUDC - Equity Related Deferrals
Regulatory Assets [Line Items]
Total Regulatory Assets	11,188	10,781
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 2,339	$ 1,876