INCOME TAXES (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2010
Current income taxes:
Federal		$ (2,350,000)	$ 6,726,000
State		(123,000)	2,495,000
TOTAL		(2,473,000)	9,221,000
Deferred income taxes:
Federal	$ 29,722,000	36,620,000	24,954,000
State	4,233,000	5,216,000	3,554,000
TOTAL	33,955,000	41,836,000	28,508,000
Investment tax credit amortization	(143,000)	(143,000)	(237,000)
TOTAL INCOME TAX EXPENSE	33,812,000	39,220,000	$ 37,492,000
Deferred tax assets and liabilities as reflected on consolidated balance sheet
NET DEFERRED TAX LIABILITIES	396,542,000	358,252,000
Deferred tax assets:
Plant related basis differences	27,347,000	25,349,000
Net operating loss	9,055,000	22,000,000
Disallowed plant costs	1,699,000	1,754,000
Gains on hedging transactions	1,195,000	1,260,000
Regulated liabilities related to income taxes	13,142,000	13,350,000
Pensions and other post-retirement benefits		1,175,000
Carry forward of income tax credit	8,675,000	6,367,000
Other	1,550,000	1,633,000
Total deferred tax assets	62,663,000	72,888,000
Deferred tax liabilities:
Depreciation, amortization and other plant related differences	382,897,000	363,337,000
Regulated assets related to income	38,615,000	37,180,000
Loss on reacquired debt	3,572,000	3,828,000
Amortization of intangibles	10,248,000	9,168,000
Pensions and other post-retirement benefits	7,112,000
Deferred construction accounting costs	5,711,000	6,082,000
Other	11,050,000	11,545,000
Total deferred tax liabilities	459,205,000	431,140,000
NET DEFERRED TAX LIABILITIES	$ 396,542,000	$ 358,252,000
Effective Income Tax Rates
Federal statutory income tax rate (as a percent)	35.00%	35.00%	35.00%
Increase (decrease) in income tax rate resulting from:
State income tax (net of federal benefit) (as a percent)	3.10%	3.10%	3.10%
Investment tax credit amortization (as a percent)	(0.20%)	(0.10%)	(0.20%)
Effect of ratemaking on property related differences (as a percent)	(1.40%)	(1.70%)	(1.10%)
Other (as a percent)	0.90%	0.60%	0.30%
Consolidated provision for income taxes	$ 33,812,000	$ 39,220,000	$ 37,492,000
Consolidated effective federal and state income tax rates (as a percent)	37.40%	36.90%	37.10%
Net operating loss carryfowards		$ 74,100,000
Net operating losses estimated to be utilized		38,000,000
Deferred tax assets estimated to be utilized	$ 13,500,000
Advanced coal investment tax credit				$ 17,700,000
Maximum tax credit utilized			$ 9,000,000
Adjustment due to change in tax accounting method om deduction and capitalization of tangible property		29,000,000
Adjustment due to change in tax accounting method on deductibility of VEBA plan activity		$ 14,000,000