REGULATORY MATTERS: Regulatory Assets and Liabilities Table (Details) - USD ($) $ in Thousands		Dec. 31, 2020	Dec. 31, 2019
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets		$ 1,558,895	$ 1,383,059
Regulatory Liabilities		768,834	782,185
Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[1]	345,788
Regulatory Liabilities	[1]	318,402
Not Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets		1,213,107
Regulatory Liabilities		450,432
Income taxes
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[2]	95,883	96,599
Income taxes | Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[1],[2]	0
Income taxes | Not Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[2]	95,883
Depreciation-related excess deferred income taxes
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[3]	178,997	183,881
Depreciation-related excess deferred income taxes | Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[1],[3]	178,997
Depreciation-related excess deferred income taxes | Not Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[3]	0
Removal costs
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[4]	182,334	185,685
Removal costs | Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[1],[4]	0
Removal costs | Not Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[4]	182,334
Investment tax credits
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities		97,627	94,806
Investment tax credits | Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[1]	0
Investment tax credits | Not Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities		97,627
Deferred revenue-AFUDC
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[5]	169,095	151,668
Deferred revenue-AFUDC | Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[1],[5]	125,088
Deferred revenue-AFUDC | Not Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[5]	44,007
Power supply costs
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[6]	15,009	48,492
Power supply costs | Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[1],[6]	8,397
Power supply costs | Not Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[6]	6,612
Other regulatory assets (liabilities)
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities		11,001	11,915
Other regulatory assets (liabilities) | Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[1]	5,920
Other regulatory assets (liabilities) | Not Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities		5,081
Tax reform accrual for future amortization [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[7]	16,893	9,139
Tax reform accrual for future amortization [Member] | Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[1],[7]	0
Tax reform accrual for future amortization [Member] | Not Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	[7]	16,893
Derivative
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities		1,995	0
Derivative | Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities		0
Derivative | Not Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities		1,995
Income taxes
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[8]	687,628	646,886
Income taxes | Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[1],[8]	0
Income taxes | Not Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[8]	687,628
Unfunded postretirement benefits
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[9]	444,470	347,935
Unfunded postretirement benefits | Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[1],[9]	0
Unfunded postretirement benefits | Not Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[9]	444,470
Pension expense deferrals
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[10]	200,686	172,637
Pension expense deferrals | Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[1],[10]	174,517
Pension expense deferrals | Not Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[10]	26,169
Energy efficiency regulatory asset
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[11]	13,225	1,465
Energy efficiency regulatory asset | Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[1],[11]	13,225
Energy efficiency regulatory asset | Not Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[11]	0
Fixed cost adjustment
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[6]	55,491	54,016
Fixed cost adjustment | Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[1],[6]	38,158
Fixed cost adjustment | Not Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[6]	17,333
Valmy Plant settlements
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[6]	103,085	107,525
Valmy Plant settlements | Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[1],[6]	103,085
Valmy Plant settlements | Not Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[6]	0
Asset retirement obligation
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[4]	19,035	18,835
Asset retirement obligation | Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[1],[4]	0
Asset retirement obligation | Not Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[4]	19,035
Long-term service agreement
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets		24,431	25,590
Long-term service agreement | Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[1]	14,729
Long-term service agreement | Not Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets		9,702
Other regulatory assets (liabilities)
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets		10,844	$ 8,170
Other regulatory assets (liabilities) | Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	[1]	2,074
Other regulatory assets (liabilities) | Not Earning a Return
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets		$ 8,770

[1] Earning a return includes either interest or a return on the investment as a component of rate base at the allowed rate of return.

[2] Represents the tax gross-up related to the depreciation-related excess deferred income taxes and investment tax credits included in this table and has a corresponding deferred tax asset disclosed in Note 2 - "Income Taxes."

[3] In 2017, income tax reform reduced deferred income tax assets and liabilities. For depreciation-related timing differences under the normalized tax accounting method, this reduction will flow back to customers under the statutorily prescribed average rate assumption method.

[4] Asset retirement obligations and removal costs are discussed in Note 14 - "Asset Retirement Obligations (ARO)."

[5] 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.

[6] This item is discussed in more detail in this Note 3 - "Regulatory Matters."

[7] Represents amount accrued under the May 2018 Idaho Tax Reform Settlement Stipulation (described below) for the future amortization of existing or future unspecified regulatory deferrals that would otherwise be a future liability recoverable from Idaho customers.

[8] Represents flow-through income tax accounting differences which have a corresponding deferred tax liability disclosed in Note 2 - "Income Taxes."

[9] Represents the unfunded obligation of Idaho Power’s pension and postretirement benefit plans, which are discussed in Note 12 - "Benefit Plans."

[10] Idaho Power records a regulatory asset for the difference between net periodic pension cost and pension cost considered for rate-making purposes relating to Idaho Power's defined benefit pension plan. In its Idaho jurisdiction, Idaho Power’s inclusion of pension costs for the establishment of retail rates is based upon contributions made to the pension plan. This regulatory asset account represents the difference between cumulative cash contributions and amounts collected in rates. Deferred costs are amortized into expense as the amounts are provided for in Idaho retail revenues.

[11] The energy efficiency asset includes both the Idaho and Oregon jurisdiction balances at December 31, 2020 and 2019.