BENEFIT PLANS: Level 4 (Details) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2021		Jun. 30, 2020		Jun. 30, 2021		Jun. 30, 2020		Dec. 31, 2021	Jul. 15, 2021
Pension Plan
Defined Benefit Plan Disclosure
Service cost		$ 13,693		$ 10,578		$ 27,101		$ 21,493
Interest cost		9,456		9,988		18,659		20,006
Expected return on plan assets		(16,024)		(14,089)		(32,045)		(28,119)
Amortization of prior service cost		1		1		3		3
Amortization of net loss		6,225		4,296		11,898		8,663
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		13,351		10,774		25,616		22,046
Regulatory deferral of net periodic benefit cost	[1]	(12,787)		(10,279)		(24,482)		(21,021)
IPUC Authorized recovered pension cost		4,289	[1]	4,289	[1]	8,577	[2]	8,577	[2]
Net periodic benefit cost recognized for financial reporting	[1],[2]	4,853		4,784		9,711		9,602
Defined Benefit Plan, Contributions by Employer						10,000
Pension Plan | Subsequent Event
Defined Benefit Plan Disclosure
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year										$ 40,000	$ 10,000
Senior Management Security Plan
Defined Benefit Plan Disclosure
Service cost		203		53		406		106
Interest cost		889		1,087		1,778		2,175
Expected return on plan assets		0		0		0		0
Amortization of prior service cost		74		73		148		145
Amortization of net loss		1,052		933		2,103		1,867
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		2,218		2,146		4,435		4,293
Net periodic benefit cost recognized for financial reporting		2,218		2,146		4,435		4,293
Postretirement Benefits Plan
Defined Benefit Plan Disclosure
Service cost		179		240		532		514
Interest cost		502		633		1,030		1,247
Expected return on plan assets		(597)		(597)		(1,198)		(1,202)
Amortization of prior service cost		12		12		24		24
Amortization of net loss		0		0		0		0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		96		288		388		583
Net periodic benefit cost recognized for financial reporting		96		288		388		583
Retirement Plan Total
Defined Benefit Plan Disclosure
Service cost		14,075		10,871		28,039		22,113
Interest cost		10,847		11,708		21,467		23,428
Expected return on plan assets		(16,621)		(14,686)		(33,243)		(29,321)
Amortization of prior service cost		87		86		175		172
Amortization of net loss		7,277		5,229		14,001		10,530
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		15,665		13,208		30,439		26,922
Regulatory deferral of net periodic benefit cost	[1]	(12,787)		(10,279)		(24,482)		(21,021)
IPUC Authorized recovered pension cost		4,289	[1]	4,289	[1]	8,577	[2]	8,577	[2]
Net periodic benefit cost recognized for financial reporting	[1],[2]	7,167		7,218		14,534		14,478
Net Periodic Benefit cost recognize in Other operations and maintenance		4,500		4,300		9,300		8,500
Net Periodic Benefit cost recognized in other expense, net		$ 2,600		$ 3,000		$ 5,200		$ 5,900

[1] Net periodic benefit costs for the pension plan are recognized for financial reporting based upon the authorization of each regulatory jurisdiction in which Idaho Power operates. Under IPUC order, the Idaho portion of net periodic benefit cost is recorded as a regulatory asset and is recognized in the income statement as those costs are recovered through rates.

[2] Of total net periodic benefit cost recognized for financial reporting, $4.5 million and $4.3 million, respectively, were recognized in "Other operations and maintenance" and $2.6 million and $3.0 million, respectively, were recognized in "Other income, net" on the condensed consolidated statements of income of the companies for the three months ended June 30, 2021 and 2020.