INCOME TAXES (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Current income taxes:
Federal		$ 1,552,000	$ (8,604,000)	$ 7,713,000
State		708,000	(2,120,000)	1,057,000
TOTAL		2,260,000	(10,724,000)	8,770,000
Deferred income taxes:
Federal		28,210,000	39,096,000	17,942,000
State		4,018,000	6,297,000	4,349,000
TOTAL		32,228,000	45,393,000	22,291,000
Investment tax credit amortization		(329,000)	(371,000)	(528,000)
TOTAL INCOME TAX EXPENSE		34,159,000	34,298,000	30,533,000
Deferred tax assets and liabilities as reflected on consolidated balance sheet
Current deferred tax assets, net		13,000,000	6,688,000
Non-current deferred tax liabilities, net		301,967,000	263,933,000
NET DEFERRED TAX LIABILITIES		288,967,000	257,245,000
Deferred tax assets:
Net operating loss		13,000,000	6,688,000
Disallowed plant costs		1,010,000	1,097,000
Gains on hedging transactions		1,389,000	1,454,000
Plant related basis differences		21,571,000	21,044,000
Regulated liabilities related to income taxes		13,871,000	13,318,000
Carry forward of income tax credit		3,722,000	16,304,000
Pensions and other post-retirement benefits		693,000
Deferred fuel costs		785,000
Other		1,477,000	891,000
Total deferred tax assets		57,518,000	60,796,000
Deferred tax liabilities:
Depreciation, amortization and other plant related differences		279,604,000	253,743,000
Regulated assets related to income		39,553,000	40,555,000
Loss on reacquired debt		4,489,000	4,288,000
Pensions and other post-retirement benefits			673,000
Amortization of intangibles		7,009,000	5,929,000
Deferred fuel costs			2,662,000
Other		15,830,000	10,191,000
Total deferred tax liabilities		346,485,000	318,041,000
NET DEFERRED TAX LIABILITIES		288,967,000	257,245,000
Payment for which recognition is deferred for tax purposes as per agreement with the IRS		26,100,000
Number of years payment be deferred for tax purposes will be recognized		50 years
Tax net operating losses		27,200,000
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)		(0.40%)	(0.40%)	(0.70%)
Effect of ratemaking on property related differences (as a percent)		(0.20%)	0.20%	(0.80%)
Effect of Medicare part D changes (as a percent)				2.70%
Other (as a percent)		0.50%	0.50%	(0.10%)
EFFECTIVE INCOME TAX RATE (as a percent)		38.00%	38.40%	39.20%
Unrecognized Tax Benefits
Unrecognized tax benefits, opening balance	906,000		359,000	906,000
Reductions to unrecognized tax benefits as a result of a lapse of the applicable statute of limitations			(359,000)	(547,000)
UNRECOGNIZED TAX BENEFITS, closing balance				359,000
Advanced coal investment tax credit		17,700,000
Maximum tax credit utilized		200,000
Investment tax credit expected to be utilized		1,800,000
Amount received from Southwest Power Administration (SWPA) as reimbursement towards estimated future lifetime replacement cost of the electrical energy and capacity lost due				26,600,000
Increase in current tax liability on recognition of taxable income				10,000,000
Amortization period of regulatory asset related to the tax benefits of cost of removal		18 years
Tax benefits of cost of removal flowed through to customers		11,100,000
Write-off of estimated portion of regulatory asset related to amortization period where rate recovery would no longer be probable	1,200,000
Tax benefits of cost of removal outstanding		$ 9,600,000