REGULATORY ASSETS & REGULATORY LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Regulatory Assets [Line Items]
Regulatory assets	$ 356,962	$ 323,434
Environmental Remediation Costs: Expended - Net
Regulatory Assets [Line Items]
Regulatory assets	47,023	42,032
Environmental Remediation Costs: Liability for Future Expenditures
Regulatory Assets [Line Items]
Regulatory assets	136,779	123,194
Deferred Asset Retirement Obligation Costs
Regulatory Assets [Line Items]
Regulatory assets	42,515	42,430
Deferred Pension and Other Postretirement Benefit Costs
Regulatory Assets [Line Items]
Regulatory assets	79,779	79,779
Deferred Gas Costs - Net
Regulatory Assets [Line Items]
Regulatory assets	0	2,701
Conservation Incentive Program Receivable
Regulatory Assets [Line Items]
Regulatory assets	19,205	2,624
Deferred Interest Rate Contracts
Regulatory Assets [Line Items]
Regulatory assets	9,379	7,631
Energy Efficiency Tracker
Regulatory Assets [Line Items]
Regulatory assets	0	496
Pipeline Supplier Service Charges
Regulatory Assets [Line Items]
Regulatory assets	3,360	3,776
Pipeline Integrity Cost
Regulatory Assets [Line Items]
Regulatory assets	4,310	4,596
AFUDC - Equity Related Deferrals
Regulatory Assets [Line Items]
Regulatory assets	11,638	11,423
Other Regulatory Assets
Regulatory Assets [Line Items]
Regulatory assets	$ 2,974	$ 2,752