SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details 4) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Fuel, Materials and Supplies
Total	$ 61,870	$ 62,239
Income taxes
longest remaining amortization period for investment tax credits	51 years
Accounting for Uncertainty in Income Taxes
No material changes to unrecognized tax benefits in other expenses	12 months
Weighted Average Number Of Shares
Basic (in shares)	42,257,000	41,852,000	40,545,000
Dilutive Securities:
Performance-based restricted stock awards (in shares)	14,500	18,222	14,991
Dividend equivalents (in shares)	6,329	9,585	12,558
Employee stock purchase plan (in shares)	1,996	3,815	7,170
Stock options (in shares)	3,160	3,240
Time-based restricted stock awards (in shares)	1,820	807
Total dilutive securities (in shares)	27,805	35,669	34,719
Diluted weighted average number of shares	42,284,000	41,887,000	40,580,000
Antidilutive Shares	128,500	128,500	74,800
Electric fuel inventory
Fuel, Materials and Supplies
Total	27,954	27,431
Natural gas inventory
Fuel, Materials and Supplies
Total	4,776	6,346
Materials and supplies
Fuel, Materials and Supplies
Total	$ 29,140	$ 28,462