ACCUMULATED OTHER COMPREHENSIVE LOSS (AOCL) - Additional Details (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Interest Charges									$ (19,906)	$ (17,872)	$ (12,550)
Other Income & Expense									3,844	5,560	3,797
Operating Expenses: Operations									(107,836)	(102,428)	(85,805)
Income Taxes	$ (10,352)	$ 2,871	$ (3,292)	$ (26,172)	$ (9,455)	$ (534)	$ (2,379)	$ (22,527)	(36,945)	(34,895)	(34,833)
Net Income	$ 22,194	$ (3,429)	$ 5,226	$ 42,587	$ 24,041	$ 974	$ 3,811	$ 37,657	$ 66,578	$ 66,483	$ 62,236
Combined statutory rate									40.00%	40.00%	41.00%
Amounts Reclassified from AOCL
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Net Income									$ 1,803	$ 659
Amounts Reclassified from AOCL | Unrealized Loss on Derivatives-Other - Interest Rate Contracts designated as cash flow hedges
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Interest Charges									46	46
Income Taxes									(23)	(19)
Net Income									23	27
Amounts Reclassified from AOCL | Unrealized Gain (Loss) on Available-for-Sale Securities
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Other Income & Expense									90	(918)
Income Taxes									(37)	370
Net Income									53	(548)
Amounts Reclassified from AOCL | Actuarial Loss on Postretirement Benefits
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Operating Expenses: Operations									2,891	1,975
Income Taxes									(1,164)	(795)
Net Income									$ 1,727	$ 1,180