SIGNIFICANT ACCOUNTING POLICIES (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accounts Receivable
Accounts receivable reserve	$ 0	$ 0
Property, Plant and Equipment
Depreciation and amortization expense	$ 135,500,000	$ 118,900,000	$ 109,400,000
Composite depreciation rate	2.10%	2.10%	2.20%
Property, Plant and Equipment [Line Items]
AFUDC debt	$ 6,800,000	$ 5,100,000	$ 8,000,000
Generator Interconnection Projects and Contributions in Aid of Construction
Contributions in aid of construction	17,400,000	19,700,000	2,600,000
Goodwill and Intangible Assets
Goodwill impairment	0
Deferred Financing Fees and Discount or Premium on Debt
Amortization of deferred financing fees and debt discounts	4,200,000	4,100,000	$ 4,100,000
Asset Retirement Obligations
Asset retirement obligations	$ 5,400,000	$ 5,900,000
Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	48 years
Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	60 years