SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) - USD ($)		1 Months Ended	12 Months Ended
	Oct. 01, 2014	Sep. 30, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Equity investment impairments			$ 0	$ 0	$ 0
PUA included in both utility revenue and cost of sales			1,100,000	1,200,000	1,100,000
Utility Plant:
Production Plant			296,000	296,000
Storage Plant			60,661,000	20,872,000
Transmission Plant			257,169,000	252,934,000
Distribution Plant			1,871,703,000	1,702,148,000
General Plant			169,464,000	163,420,000
Other Plant			1,855,000	1,855,000
Utility Plant in Service			2,361,148,000	2,141,525,000
Construction Work in Progress			62,986,000	69,714,000
Total Utility Plant			2,424,134,000	2,211,239,000
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
ARO as of January 1,			57,219,000	41,976,000
Accretion			1,908,000	1,658,000
Additions			1,098,000	621,000
Settlements			(1,551,000)	(1,110,000)
Revisions in Estimated Cash Flows			0	14,074,000
ARO as of December 31,			$ 58,674,000	$ 57,219,000	$ 41,976,000
Composite annual rate for all depreciable utility property			2.20%	2.20%	2.20%
Composite depreciation rate	2.10%	2.40%
Interest capitalized			$ 5,300,000	$ 2,800,000	$ 4,400,000