INCOME TAXES (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2010	Dec. 31, 2011 Y	Dec. 31, 2010	Dec. 31, 2009	Sep. 07, 2010
Current income taxes:
Federal		$ (8,604,000)	$ 7,713,000	$ 3,987,000
State		(2,120,000)	1,057,000	572,000
TOTAL		(10,724,000)	8,770,000	4,559,000
Deferred income taxes:
Federal		39,096,000	17,942,000	13,854,000
State		6,297,000	4,349,000	1,973,000
TOTAL		45,393,000	22,291,000	15,827,000
Investment tax credit amortization		(371,000)	(528,000)	(504,000)
TOTAL INCOME TAX EXPENSE		34,298,000	30,533,000	19,882,000
Deferred tax assets and liabilities as reflected on consolidated balance sheet
Current deferred tax assets, net		6,688,000
Non-current deferred tax liabilities, net		263,933,000	212,003,000
NET DEFERRED TAX LIABILITIES		257,245,000	212,003,000
Deferred tax assets:
Net operating loss		6,688,000
Disallowed plant costs		1,097,000	1,127,000
Alternative minimum tax		261,000
Gains on hedging transactions		1,454,000	1,518,000
Plant related basis differences		21,044,000	21,105,000
Regulated liabilities related to income taxes		13,318,000	13,702,000
Pensions and other post-retirement benefits			1,588,000
Carry forward of income tax credit		16,304,000	12,596,000
Income received- deferred		598,000	10,044,000
Other		32,000	2,262,000
Total deferred tax assets		60,796,000	63,942,000
Deferred tax liabilities:
Depreciation, amortization and other plant related differences		253,743,000	216,685,000
Regulated assets related to income		40,555,000	41,107,000
Loss on reacquired debt		4,288,000	3,996,000
Pensions and other post-retirement benefits		673,000
Deferred ice storm expenses		1,413,000	2,957,000
Deferred fuel costs		2,662,000	1,965,000
Amortization of intangibles		5,929,000	4,850,000
Other		8,778,000	4,385,000
Total deferred tax liabilities		318,041,000	275,945,000
NET DEFERRED TAX LIABILITIES		257,245,000	212,003,000
Tax net operating losses		25,100,000
Number of years to which tax net operating losses may be carried back		2
Maximum tax credit utilized		200,000
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
Effective Income Tax Rates
Federal statutory income tax rate (as a percent)		35.00%	35.00%	35.00%
Increase 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%)	(0.70%)	(0.80%)
Effect of ratemaking on property related differences (as a percent)		0.20%	(0.80%)	(3.60%)
Effect of Medicare part D changes (as a percent)			2.70%
Other (as a percent)		0.50%	(0.10%)	(1.20%)
Effective tax rate (as a percent)		38.40%	39.20%	32.50%
Unrecognized Tax Benefits
Unrecognized tax benefits, opening balance	906,000	359,000	906,000	2,176,000
Reductions to unrecognized tax benefits as a result of a lapse of the applicable statute of limitations		(359,000)	(547,000)	(1,270,000)
Unrecognized tax benefits, closing balance		0	359,000	906,000
Interest or penalties related to unrecognized tax benefits		0	100,000	0
Coal investment tax credit granted to Iatan 2 plant				125,000,000
Advanced coal investment tax credit					17,700,000
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
One-time non-cash charge	$ 2,100,000