INCOME TAXES (The Significant Components of Deferred Income Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Gross Deferred Income Tax Assets
Capital Lease Obligations	$ 35,000	$ 27,000
Net Operating Loss Carryforwards	129,000	156,000
Customer Advances and Contributions in Aid of Construction	20,000	20,000
Alternative Minimum Tax Credit	25,000	24,000
Accrued Postretirement Benefits	23,000	23,000
Emission Allowance Inventory	9,000	9,000
Investment Tax Credit Carryforward	32,000	32,000
Other	60,000	53,000
Total Gross Deferred Income Tax Assets	333,000	344,000
Deferred Tax Assets Valuation Allowance	0	(4,000)
Gross Deferred Income Tax Liabilities
Plant, Net	(774,000)	(750,000)
Capital Lease Assets, Net	(24,000)	(12,000)
Pensions	(26,000)	(27,000)
Other	(38,000)	(19,000)
Total Gross Deferred Income Tax Liabilities	(862,000)	(808,000)
Deferred Tax Liabilities, Net, Noncurrent	$ 529,148	$ 468,024