BENEFIT PLANS: Defined Benefit Plan, Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017
Defined Benefit Plan Disclosure
Net periodic benefit cost		$ 10,976	$ 15,781	[1]	$ 11,194
Pension Plan
Defined Benefit Plan Disclosure
Service cost		34,061	37,836		33,742
Interest cost		42,312	38,833		38,957
Expected return on plan assets		48,623	52,302		45,138
Amortization of net loss		(13,564)	(13,558)		(13,190)
Amortization of prior service cost		6	6		28
Net periodic benefit cost		41,320	37,931		40,779
Regulatory deferral of net periodic benfit cost	[2]	39,379	36,153		38,699
Previously deferred pension cost recognized	[2]	17,154	17,154		17,154
Net periodic benefit cost recognized for financial reporting	[2],[3]	19,095	18,932		19,234
Pension and SMSP
Defined Benefit Plan Disclosure
NetServiceCostDefinedBenefitPlan		15,100	15,200		16,200
otherexpensedefinedbenefitplanbenefitcost		11,000	11,600		11,200
Senior Management Security Plan
Defined Benefit Plan Disclosure
Service cost		(181)	(316)		759
Interest cost		4,575	4,248		4,315
Expected return on plan assets		0	0		0
Amortization of net loss		(2,533)	(3,788)		(2,963)
Amortization of prior service cost		96	98		127
Net periodic benefit cost		7,023	7,818		8,164
Regulatory deferral of net periodic benfit cost		0	0		0
Net periodic benefit cost recognized for financial reporting	[3]	$ 7,023	$ 7,818		$ 8,164

[1] The 2018 pension and postretirement non-service costs includes $4.2 million of expense for a temporary deviation from the cost-sharing provisions of the substantive postretirement plan as described in Note 12 - "Benefit Plans."

[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 $15.1 million, $15.2 million, and $16.2 million respectively, was recognized in "Other operations and maintenance" and $11.0 million, and $11.6 million, and $11.2 million respectively, was recognized in "Other expense, net" on the consolidated statements of income of the companies for the twelve months ended December 31, 2019, 2018, and 2017.