PROPERTY, PLANT AND EQUIPMENT (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Property, Plant and Equipment
Plant and property, Gross	$ 2,063,208,000	$ 2,001,142,000
Plant and property, Gross	34,984,000	32,264,000
Plant and property, Gross	66,918,000	63,581,000
Plant and property, Gross	11,540,000	11,128,000
Less accumulated depreciation and amortization	637,139,000	598,363,000
Construction work in progress	24,141,000	9,337,000
Total plant and property, Net	1,563,652,000	1,519,089,000
Provision for depreciation	59,920,000	53,941,000	48,488,000
Amortization	7,445,000	8,347,000	5,159,000
Total depreciation and amortization	67,365,000	62,288,000	53,647,000
Annual depreciation rates (as a percent)	2.90%	2.90%	3.00%
Regulated
Property, Plant and Equipment
Amortization	6,600,000	7,500,000	4,500,000
Non-Regulated
Property, Plant and Equipment
Provision for depreciation	1,807,000	1,641,000	1,443,000
Annual depreciation rates (as a percent)	5.40%	5.30%	5.00%
Electric plant | Regulated
Property, Plant and Equipment
Plant and property, Gross	2,063,208,000	2,001,142,000
Less accumulated depreciation and amortization	610,084,000	575,061,000
Plant and property net of depreciation and amortization	1,453,124,000	1,426,081,000
Construction work in progress	23,494,000	9,214,000
Total plant and property, Net	1,476,618,000	1,435,295,000
Electric plant | Intangible property, primarily capitalized software and investments in facility upgrades | Regulated
Property, Plant and Equipment
Plant and property, Gross	22,100,000	20,100,000
Less accumulated depreciation and amortization	9,900,000	9,200,000
Production | Regulated
Property, Plant and Equipment
Plant and property, Gross	1,023,154,000	1,015,040,000
Annual depreciation rates (as a percent)	2.10%	2.00%	2.20%
Transmission | Regulated
Property, Plant and Equipment
Plant and property, Gross	232,390,000	220,514,000
Annual depreciation rates (as a percent)	2.30%	2.40%	2.40%
Distribution | Regulated
Property, Plant and Equipment
Plant and property, Gross	719,731,000	682,175,000
Annual depreciation rates (as a percent)	3.60%	3.60%	3.60%
General | Regulated
Property, Plant and Equipment
Plant and property, Gross	87,933,000	83,413,000
Less accumulated depreciation and amortization	37,300,000	30,700,000
Amortization	6,600,000	7,500,000	4,500,000
Annual depreciation rates (as a percent)	6.10%	6.20%	6.10%
Gas plant | Regulated
Property, Plant and Equipment
Plant and property, Gross	66,918,000	63,581,000
Less accumulated depreciation and amortization	10,851,000	8,994,000
Plant and property net of depreciation and amortization	56,067,000	54,587,000
Construction work in progress	79,000	6,000
Total plant and property, Net	56,146,000	54,593,000
Provision for depreciation	3,485,000	3,046,000	2,072,000
Annual depreciation rates (as a percent)	5.50%	5.10%	3.70%
Water plant | Regulated
Property, Plant and Equipment
Plant and property, Gross	11,540,000	11,128,000
Less accumulated depreciation and amortization	4,158,000	3,855,000
Plant and property net of depreciation and amortization	7,382,000	7,273,000
Construction work in progress	126,000	40,000
Total plant and property, Net	7,508,000	7,313,000
Annual depreciation rates (as a percent)	2.70%	2.70%	2.70%
Other | Non-Regulated
Property, Plant and Equipment
Construction work in progress	442,000	77,000
Total plant and property, Net	23,380,000	21,888,000
Fiber | Non-Regulated
Property, Plant and Equipment
Plant and property, Gross	34,984,000	32,264,000
Less accumulated depreciation and amortization	12,046,000	10,453,000
Plant and property net of depreciation and amortization	22,938,000	21,811,000
Electric and Water | Regulated
Property, Plant and Equipment
Provision for depreciation	$ 54,628,000	$ 49,254,000	$ 44,973,000
Annual depreciation rates (as a percent)	2.70%	2.80%	2.90%