BENEFIT PLANS: Level 4 (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Pension Plan
Defined Benefit Plan Disclosure
Service cost		$ 13,551	$ 10,747	$ 40,652	$ 32,240
Interest cost		9,329	10,004	27,988	30,010
Expected return on plan assets		(16,023)	(14,060)	(48,068)	(42,179)
Amortization of prior service cost		2	1	5	4
Amortization of net loss		5,949	4,331	17,847	12,994
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		12,808	11,023	38,424	33,069
Regulatory deferral of net periodic benefit cost	[1]	(12,240)	(10,511)	(36,722)	(31,532)
IPUC Authorized recovered pension cost	[1]	4,288	4,288	12,865	12,865
Net periodic benefit cost recognized for financial reporting	[1],[2]	4,856	4,800	14,567	14,402
Defined Benefit Plan, Contributions by Employer				40,000
Senior Management Security Plan
Defined Benefit Plan Disclosure
Service cost		203	54	609	160
Interest cost		889	1,087	2,667	3,262
Expected return on plan assets		0	0	0	0
Amortization of prior service cost		74	73	222	218
Amortization of net loss		1,052	933	3,155	2,800
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		2,218	2,147	6,653	6,440
Net periodic benefit cost recognized for financial reporting		2,218	2,147	6,653	6,440
Postretirement Benefits Plan
Defined Benefit Plan Disclosure
Service cost		265	258	797	772
Interest cost		514	623	1,544	1,870
Expected return on plan assets		(598)	(601)	(1,796)	(1,803)
Amortization of prior service cost		11	11	35	35
Amortization of net loss		0	0	0	0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		192	291	580	874
Net periodic benefit cost recognized for financial reporting		192	291	580	874
Retirement Plan Total
Defined Benefit Plan Disclosure
Service cost		14,019	11,059	42,058	33,172
Interest cost		10,732	11,714	32,199	35,142
Expected return on plan assets		(16,621)	(14,661)	(49,864)	(43,982)
Amortization of prior service cost		87	85	262	257
Amortization of net loss		7,001	5,264	21,002	15,794
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		15,218	13,461	45,657	40,383
Regulatory deferral of net periodic benefit cost	[1]	(12,240)	(10,511)	(36,722)	(31,532)
IPUC Authorized recovered pension cost	[1]	4,288	4,288	12,865	12,865
Net periodic benefit cost recognized for financial reporting	[1],[2]	7,266	7,238	21,800	21,716
Net Periodic Benefit cost recognize in Other operations and maintenance		4,700	4,300	13,900	12,800
Net Periodic Benefit cost recognized in other expense, net		$ 2,600	$ 2,900	$ 7,900	$ 8,900

[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, $4.7 million and $4.3 million, respectively, were recognized in "Other operations and maintenance" and $2.6 million and $2.9 million, respectively, were recognized in "Other income, net" on the condensed consolidated statements of income of the companies for the three months ended September 30, 2021 and 2020.