ACCUMULATED OTHER COMPREHENSIVE LOSS (AOCL) - Additional Details (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Operating Expenses: Operations									$ 95,609	$ 107,836	$ 102,428
Total Income Tax Expense	$ 12,554	$ (2,007)	$ 1,415	$ 27,404	$ 10,352	$ (2,871)	$ 3,292	$ 26,172	39,366	36,945	$ 34,895
Losses from reclassifications for the period net of tax									$ 1,292	$ 1,803
Combined statutory rate									40.00%	40.00%	40.00%
Unrealized Loss on Derivatives-Other - Interest Rate Contracts designated as cash flow hedges
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Losses from reclassifications for the period net of tax									$ 27	$ 23
Unrealized Gain (Loss) on Available-for-Sale Securities
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Losses from reclassifications for the period net of tax									(225)	53
Actuarial Loss on Postretirement Benefits
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Losses from reclassifications for the period net of tax									1,490	1,727
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]
Total Income Tax Expense									(19)	(23)
Net income (loss)									27	23
Amounts Reclassified from AOCL | Unrealized Gain (Loss) on Available-for-Sale Securities
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Other Income & Expense									(377)	90
Total Income Tax Expense									152	(37)
Net income (loss)									(225)	53
Amounts Reclassified from AOCL | Actuarial Loss on Postretirement Benefits
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Operating Expenses: Operations									2,490	2,891
Total Income Tax Expense									(1,000)	(1,164)
Net income (loss)									1,490	1,727
Interest rate contract | 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