REGULATORY ASSETS AND LIABILITIES - Liabilities (Details) - USD ($) $ in Thousands	1 Months Ended
	Sep. 30, 2014	Dec. 31, 2016	Dec. 31, 2015
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities		$ 49,121	$ 42,841
Regulatory assets		410,746	323,434
Deferred Gas Costs - Net
Regulatory Liabilities [Line Items]
Regulatory assets		0	2,701
Excess Plant Removal Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities		28,226	32,644
Annual amortization reflected in depreciation expense	$ 1,100
Deferred Gas Costs - Net
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities		17,800	0
Societal Benefit Costs Payable
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities		$ 3,095	$ 10,197