BENEFIT PLANS: Level 4 (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2020		Sep. 30, 2019		Sep. 30, 2020		Sep. 30, 2019
Pension Plan
Defined Benefit Plan Disclosure
Service cost	$ 10,747		$ 8,516		$ 32,240		$ 25,546
Interest cost	10,004		10,578		30,010		31,734
Expected return on plan assets	(14,060)		(12,156)		(42,179)		(36,467)
Amortization of prior service cost	1		1		4		4
Amortization of net loss	4,331		3,391		12,994		10,173
Net periodic benefit cost	11,023		10,330		33,069		30,990
Regulatory deferral of net periodic benefit cost	(10,511)	[1]	(9,845)	[1]	(31,532)	[2]	(29,534)	[2]
IPUC Authorized recovered pension cost	4,288	[1]	4,288	[1]	12,865	[2]	12,865	[2]
Net periodic benefit cost recognized for financial reporting	4,800	[1],[3]	4,773	[1],[3]	14,402	[2],[4]	14,321	[2],[4]
Defined Benefit Plan, Contributions by Employer					40,000
Senior Management Security Plan
Defined Benefit Plan Disclosure
Service cost	54		(46)		160		(136)
Interest cost	1,087		1,143		3,262		3,432
Expected return on plan assets	0		0		0		0
Amortization of prior service cost	73		24		218		72
Amortization of net loss	933		634		2,800		1,900
Net periodic benefit cost	2,147		1,755		6,440		5,268
Net periodic benefit cost recognized for financial reporting	2,147	[3]	1,755	[3]	6,440	[4]	5,268	[4]
Postretirement Benefits Plan
Defined Benefit Plan Disclosure
Service cost	258		213		772		640
Interest cost	623		748		1,870		2,242
Expected return on plan assets	(601)		(555)		(1,803)		(1,665)
Amortization of prior service cost	11		11		35		35
Amortization of net loss	0		0		0		0
Net periodic benefit cost	291		417		874		1,252
Net periodic benefit cost recognized for financial reporting	291		417		874		1,252
Retirement Plan Total
Defined Benefit Plan Disclosure
Service cost	11,059		8,683		33,172		26,050
Interest cost	11,714		12,469		35,142		37,408
Expected return on plan assets	(14,661)		(12,711)		(43,982)		(38,132)
Amortization of prior service cost	85		36		257		111
Amortization of net loss	5,264		4,025		15,794		12,073
Net periodic benefit cost	13,461		12,502		40,383		37,510
Regulatory deferral of net periodic benefit cost	(10,511)	[1]	(9,845)	[1]	(31,532)	[2]	(29,534)	[2]
IPUC Authorized recovered pension cost	4,288	[1]	4,288	[1]	12,865	[2]	12,865	[2]
Net periodic benefit cost recognized for financial reporting	7,238	[1],[3]	6,945	[1],[3]	21,716	[2],[4]	20,841	[2],[4]
Net Periodic Benefit cost recognize in Other operations and maintenance	4,300		4,200		12,800		12,600
Net Periodic Benefit cost recognized in other expense, net	$ 3,000		$ 2,700		$ 8,900		$ 8,200

[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] 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.

[3] Of total net periodic benefit cost recognized for financial reporting, $4.3 million and $4.2 million, respectively, were recognized in "Other operations and maintenance" and $3.0 million and $2.7 million, respectively, were recognized in "Other expense, net" on the condensed consolidated statements of income of the companies for the three months ended September 30, 2020 and 2019.

[4] Of total net periodic benefit cost recognized for financial reporting, $12.8 million and $12.6 million, respectively, were recognized in "Other operations and maintenance" and $8.9 million and $8.2 million, respectively, were recognized in "Other expense, net" on the condensed consolidated statements of income of the companies for the nine months ended September 30, 2020 and 2019.