PROPERTY, PLANT AND EQUIPMENT (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Property, Plant and Equipment
Plant and property, Gross	$ 37,983,000	$ 34,984,000
Plant and property, Gross	69,851,000	66,918,000
Less accumulated depreciation and amortization	682,737,000	637,139,000
Construction work in progress	56,347,000	24,141,000
Total plant and property, Net	1,657,632,000	1,563,652,000
Provision for depreciation	62,607,000	59,920,000	53,941,000
Amortization	1,041,000	7,445,000	8,347,000
Total depreciation and amortization	63,648,000	67,365,000	62,288,000
Annual depreciation rates (as a percent)	2.90%	2.90%	2.90%
Regulated
Property, Plant and Equipment
Amortization		6,600,000	7,500,000
Non-Regulated
Property, Plant and Equipment
Provision for depreciation	1,538,000	1,807,000	1,641,000
Annual depreciation rates (as a percent)	4.20%	5.40%	5.30%
Electric plant | Regulated
Property, Plant and Equipment
Plant and property, Gross	2,163,872,000	2,063,208,000
Less accumulated depreciation and amortization	651,627,000	610,084,000
Plant and property net of depreciation and amortization	1,512,245,000	1,453,124,000
Construction work in progress	55,957,000	23,494,000
Total plant and property, Net	1,568,202,000	1,476,618,000
Electric plant | Intangible property, primarily capitalized software and investments in facility upgrades | Regulated
Property, Plant and Equipment
Plant and property, Gross	36,400,000	22,100,000
Less accumulated depreciation and amortization	10,700,000	9,900,000
Production | Regulated
Property, Plant and Equipment
Plant and property, Gross	1,034,114,000	1,023,154,000
Annual depreciation rates (as a percent)	2.00%	2.10%	2.00%
Transmission | Regulated
Property, Plant and Equipment
Plant and property, Gross	251,769,000	232,390,000
Annual depreciation rates (as a percent)	2.40%	2.30%	2.40%
Distribution | Regulated
Property, Plant and Equipment
Plant and property, Gross	766,026,000	719,731,000
Annual depreciation rates (as a percent)	3.60%	3.60%	3.60%
General | Regulated
Property, Plant and Equipment
Plant and property, Gross	111,963,000	87,933,000
Less accumulated depreciation and amortization	37,300,000	37,300,000
Amortization		6,600,000	7,500,000
Annual depreciation rates (as a percent)	5.90%	6.10%	6.20%
Gas plant | Regulated
Property, Plant and Equipment
Plant and property, Gross	69,851,000	66,918,000
Less accumulated depreciation and amortization	12,940,000	10,851,000
Plant and property net of depreciation and amortization	56,911,000	56,067,000
Construction work in progress	184,000	79,000
Total plant and property, Net	57,095,000	56,146,000
Provision for depreciation	3,602,000	3,485,000	3,046,000
Annual depreciation rates (as a percent)	5.40%	5.50%	5.10%
Water plant | Regulated
Property, Plant and Equipment
Plant and property, Gross	12,316,000	11,540,000
Less accumulated depreciation and amortization	4,440,000	4,158,000
Plant and property net of depreciation and amortization	7,876,000	7,382,000
Construction work in progress	1,000	126,000
Total plant and property, Net	7,877,000	7,508,000
Annual depreciation rates (as a percent)	2.70%	2.70%	2.70%
Other | Non-Regulated
Property, Plant and Equipment
Construction work in progress	205,000	442,000
Total plant and property, Net	24,458,000	23,380,000
Fiber | Non-Regulated
Property, Plant and Equipment
Plant and property, Gross	37,983,000	34,984,000
Less accumulated depreciation and amortization	13,730,000	12,046,000
Plant and property net of depreciation and amortization	24,253,000	22,938,000
Electric and Water | Regulated
Property, Plant and Equipment
Provision for depreciation	$ 57,467,000	$ 54,628,000	$ 49,254,000
Annual depreciation rates (as a percent)	2.80%	2.70%	2.80%