REGULATORY ASSETS AND REGULATORY LIABILITIES - Regulatory Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 491,629	$ 469,224
Environmental Remediation Costs: Expended - Net
Regulatory Assets [Line Items]
Total Regulatory Assets	110,529	100,327
Environmental Remediation Costs: Liability for Future Expenditures
Regulatory Assets [Line Items]
Total Regulatory Assets	161,263	171,696
Deferred Asset Retirement Obligation Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	42,622	42,368
Deferred Pension and Other Postretirement Benefit Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	78,211	78,211
Deferred Gas Costs
Regulatory Assets [Line Items]
Total Regulatory Assets	54,351	16,838
Conservation Incentive Program Receivable
Regulatory Assets [Line Items]
Total Regulatory Assets	12,118	26,652
Societal Benefit Costs Receivable
Regulatory Assets [Line Items]
Total Regulatory Assets	2,699	2,484
Deferred Interest Rate Contracts
Regulatory Assets [Line Items]
Total Regulatory Assets	6,006	7,028
Energy Efficiency Tracker
Regulatory Assets [Line Items]
Total Regulatory Assets	1,832	2,094
Pipeline Supplier Service Charges
Regulatory Assets [Line Items]
Total Regulatory Assets	686	708
Pipeline Integrity Cost
Regulatory Assets [Line Items]
Total Regulatory Assets	4,977	5,280
AFUDC - Equity Related Deferrals
Regulatory Assets [Line Items]
Total Regulatory Assets	12,899	12,785
Other Regulatory Assets
Regulatory Assets [Line Items]
Total Regulatory Assets	$ 3,436	$ 2,753