SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Detail 4) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Fuel, Materials and Supplies
Total	$ 60,950	$ 57,799
Income taxes
Longest remaining amortization period for investment tax credits	55 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)	43,671,000	43,291,000	42,781,000
Dilutive Securities:
Performance-based restricted stock awards (in shares)	19,890	8,809	12,142
Employee stock purchase plan (in shares)	1,249	3,422	1,729
Stock options (in shares)			61
Time-based restricted stock awards (in shares)	25,523	10,666	7,907
Total dilutive securities (in shares)	46,662	22,897	21,839
Diluted weighted average number of shares	43,718,000	43,314,000	42,803,000
Antidilutive Shares	20,289	25,259	107,100
Electric fuel inventory
Fuel, Materials and Supplies
Total	$ 30,185	$ 26,454
Natural gas inventory
Fuel, Materials and Supplies
Total	3,868	5,040
Materials and supplies
Fuel, Materials and Supplies
Total	$ 26,897	$ 26,305