PROPERTY, PLANT AND EQUIPMENT AND JOINTLY-OWNED PROJECTS: Property, Plant and Equipmnt Level 4 (Details)		12 Months Ended
		Dec. 31, 2019 USD ($) MWh	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	May 31, 2018 USD ($)	Jun. 01, 2017 USD ($)
Public Utility, Property, Plant and Equipment
Production		$ 2,535,938,000	$ 2,654,201,000
Transmission		1,220,703,000	1,201,092,000
Distribution		1,882,136,000	1,792,284,000
General and Other		474,790,000	456,279,000
Utility plant in service		6,113,567,000	6,103,856,000
Accumulated provision for depreciation		(2,155,783,000)	(2,210,781,000)
Utility plant in service - net		$ 3,957,784,000	$ 3,893,075,000
Average depreciation rate		2.87%	2.84%	2.90%
Idaho Energy Resources Co. Coal purchases from Bridger Coal Company		$ 156,872,000	$ 133,198,000	$ 145,829,000
Regulatory Liabilities		782,185,000	764,877,000
Construction work in progress		$ 552,499,000	$ 480,259,000
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		$ 745,096,000
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress		4,622,000
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation		$ 353,254,000
Jointly Owned Utility Plant, Proportionate Ownership Share		33.00%
Nameplate Capacity (in MW) | MWh	[1]	771
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		$ 82,501,000
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress		12,000
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation		$ 78,411,000
Jointly Owned Utility Plant, Proportionate Ownership Share		10.00%
Nameplate Capacity (in MW) | MWh	[1]	64
Valmy Plant
Public Utility, Property, Plant and Equipment
Jointly Owned Utility Plant, Name		North Valmy unit 2
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service		$ 252,921,000
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress		217,000
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation		$ 166,419,000
Jointly Owned Utility Plant, Proportionate Ownership Share		50.00%
Nameplate Capacity (in MW) | MWh	[1]	145
Production
Public Utility, Property, Plant and Equipment
Average depreciation rate		3.19%	3.10%
Transmission
Public Utility, Property, Plant and Equipment
Average depreciation rate		1.89%	1.89%
Distribution
Public Utility, Property, Plant and Equipment
Average depreciation rate		2.25%	2.24%
General and Other
Public Utility, Property, Plant and Equipment
Average depreciation rate		6.17%	6.40%
Bridger Coal Company
Public Utility, Property, Plant and Equipment
Idaho Energy Resources Co. Coal purchases from Bridger Coal Company		$ 73,600,000	$ 81,800,000	86,400,000
Idaho Power Company
Public Utility, Property, Plant and Equipment
Utility plant in service		6,113,567,000	6,103,856,000
Accumulated provision for depreciation		(2,155,783,000)	(2,210,781,000)
Utility plant in service - net		3,957,784,000	3,893,075,000
Idaho Energy Resources Co. Coal purchases from Bridger Coal Company		156,872,000	133,198,000	145,829,000
Construction work in progress		$ 552,499,000	480,259,000
Ida-West Energy
Public Utility, Property, Plant and Equipment
Jointly Owned Utility Plant, Proportionate Ownership Share		50.00%
Marysville Hydro Partners
Public Utility, Property, Plant and Equipment
Utility plant in service - net		$ 18,000,000
Ida-West | Idaho Power Company
Public Utility, Property, Plant and Equipment
Power purchased from Ida-West		8,600,000	9,700,000	$ 9,800,000
Hells Canyon Complex | Idaho Power Company
Public Utility, Property, Plant and Equipment
Construction work in progress		326,000,000.0
Hydroelectric plan net book value [Member] | Marysville Hydro Partners
Public Utility, Property, Plant and Equipment
Utility plant in service - net		14,700,000	15,200,000
IPUC authorized AFUDC Collection HCC Relicensing - Gross | Hells Canyon Complex | Idaho Power Company
Public Utility, Property, Plant and Equipment
Regulatory Liabilities		8,800,000			$ 10,700,000	$ 10,700,000
IPUC authorized AFUDC Collection HCC Relicensing - Net | Hells Canyon Complex | Idaho Power Company
Public Utility, Property, Plant and Equipment
Regulatory Liabilities		6,500,000
Deferred revenue-AFUDC
Public Utility, Property, Plant and Equipment
Regulatory Liabilities	[2]	$ 151,668,000	$ 135,146,000

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

[2] Idaho Power is collecting revenue in the Idaho jurisdiction for AFUDC on HCC relicensing costs but is deferring revenue recognition of the amounts collected until the license is issued and the asset is placed in service under the new license.