INCOME TAXES (The Significant Components of Deferred Income Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Gross Deferred Income Tax Assets
Capital Lease Obligations	$ 10,000	$ 35,000
Operating Loss Carryforwards, Net	56,000	129,000
Customer Advances and Contributions in Aid of Construction	14,000	20,000
Alternative Minimum Tax Credit	26,000	25,000
Other Postretirement Benefits	15,000	23,000
Emission Allowance Inventory	3,000	9,000
Investment Tax Credit Carryforward	34,000	32,000
Income Taxes Recoverable Through Future Rates	88,000	0
Other	47,000	60,000
Total Gross Deferred Income Tax Assets	293,000	333,000
Deferred Tax Assets Valuation Allowance	0	0
Gross Deferred Income Tax Liabilities
Plant, Net	(518,000)	(774,000)
Plant Abandonments	(21,000)	0
Capital Lease Assets, Net	(5,000)	(24,000)
Pensions	(16,000)	(26,000)
Income Taxes Payable Through Future Rates	(10,000)	0
Other	(23,000)	(38,000)
Total Gross Deferred Income Tax Liabilities	(593,000)	(862,000)
Deferred Tax Liabilities, Net, Noncurrent	$ (300,258)	$ (529,148)