REGULATORY ASSETS & REGULATORY LIABILITIES (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 335,009	$ 357,160
Environmental Remediation Costs: Expended - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	28,057	29,540
Environmental Remediation Costs: Liability for Future Expenditures
Regulatory Assets [Line Items]
Total Regulatory Assets	124,761	124,308
Deferred Asset Retirement Obligation Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	31,604	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	20,291	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	8,330	7,325
Energy Efficiency Tracker
Regulatory Assets [Line Items]
Total Regulatory Assets	1,782	11,247
Pipeline Supplier Service Charges
Regulatory Assets [Line Items]
Total Regulatory Assets	5,025	5,441
Pipeline Integrity Cost
Regulatory Assets [Line Items]
Total Regulatory Assets	3,439	3,431
AFUDC - Equity Related Deferrals
Regulatory Assets [Line Items]
Total Regulatory Assets	10,987	10,781
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 1,693	$ 1,876