SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details 4) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fuel, Materials and Supplies
Total	$ 57,799	$ 48,811
Income taxes
Longest remaining amortization period for investment tax credits	50 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)	43,291,000	42,781,000	42,257,000
Dilutive Securities:
Performance-based restricted stock awards (in shares)	8,809	12,142	14,500
Dividend equivalents (in shares)			6,329
Employee stock purchase plan (in shares)	3,422	1,729	1,996
Stock options (in shares)		61	3,160
Time-based restricted stock awards (in shares)	10,666	7,907	1,820
Total dilutive securities (in shares)	22,897	21,839	27,805
Diluted weighted average number of shares	43,314,000	42,803,000	42,284,000
Antidilutive Shares	25,259	107,100	128,500
Electric fuel inventory
Fuel, Materials and Supplies
Total	26,454	17,003
Natural gas inventory
Fuel, Materials and Supplies
Total	5,040	3,584
Materials and supplies
Fuel, Materials and Supplies
Total	$ 26,305	$ 28,224