PROPERTY, PLANT AND EQUIPMENT AND JOINTLY-OWNED PROJECTS: Property, Plant and Equipmnt Level 4 (Details) (USD $)	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010	Dec. 31, 2009
Public Utility, Property, Plant and Equipment
Production	$ 1,832,287,000		$ 1,792,305,000
Transmission	871,784,000		855,202,000
Distribution	1,434,925,000		1,377,239,000
General and Other	327,877,000		307,308,000
Utility plant in service	4,466,873,000		4,332,054,000
Accumulated provision for depreciation	(1,677,609,000)		(1,614,013,000)
Utility plant in service - net	2,789,264,000		2,718,041,000
Average depreciation rate	2.83%		2.84%	2.81%
Sale of transmission-related assets	0		18,982,000	0
Fuel expense	131,542,000		159,673,000	149,566,000
Jim Bridger Plant
Public Utility, Property, Plant and Equipment
Jointly Owned Utility Plant, Name	Jim Bridger Units 1-4
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	539,294,000
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	8,334,000
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	276,375,000
Jointly Owned Utility Plant, Proportionate Ownership Share	33.00%
Nameplate Capacity (in MW)	771	[1]
Boardman Plant
Public Utility, Property, Plant and Equipment
Jointly Owned Utility Plant, Name	Boardman
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	79,714,000
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	940,000
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	53,843,000
Jointly Owned Utility Plant, Proportionate Ownership Share	10.00%
Nameplate Capacity (in MW)	64	[1]
Valmy Plant
Public Utility, Property, Plant and Equipment
Jointly Owned Utility Plant, Name	Valmy Units 1 and 2
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	350,582,000
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	7,352,000
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	202,811,000
Jointly Owned Utility Plant, Proportionate Ownership Share	50.00%
Nameplate Capacity (in MW)	284	[1]
Production
Public Utility, Property, Plant and Equipment
Average depreciation rate	2.22%		2.23%
Transmission
Public Utility, Property, Plant and Equipment
Average depreciation rate	2.06%		2.03%
Distribution
Public Utility, Property, Plant and Equipment
Average depreciation rate	3.12%		3.13%
General and Other
Public Utility, Property, Plant and Equipment
Average depreciation rate	7.32%		7.41%
Bridger Coal Company
Public Utility, Property, Plant and Equipment
Fuel expense	65,000,000		76,000,000	66,000,000
Ida-West Energy
Public Utility, Property, Plant and Equipment
Jointly Owned Utility Plant, Proportionate Ownership Share	50.00%
Power purchased from related parties	$ 9,000,000		$ 8,000,000	$ 9,000,000

[1] Idaho Power’s share of nameplate capacity.