BENEFIT PLANS: Benefit Plans Narrative (Details) - USD ($)		11 Months Ended	12 Months Ended
		Dec. 31, 2091	Dec. 31, 2026	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023		Dec. 31, 2022		Dec. 31, 2021
Defined Benefit Plan, Net Periodic Benefit Cost
Net periodic benefit cost	[1]					$ 6,513,000		$ 9,196,000		$ 15,249,000
Fair Value, All Levels Transfers, Amount						0		0
Employer contributions						9,800,000		8,800,000		8,200,000
Deferred Tax Liabilities, Regulatory Assets						29,742,000		33,116,000
Pension Plan
Defined Benefit Plan Disclosure
Fair value of plan assets						917,513,000		839,728,000		984,464,000
Defined Benefit Plan, Net Periodic Benefit Cost
Net periodic benefit cost						19,398,000		31,626,000		51,231,000
Amortization of net loss						0		(12,273,000)		(23,796,000)
Amortization of prior service cost						$ 6,000		$ 6,000		$ 6,000
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate						5.45%		3.05%		2.80%
Employer contributions						$ 48,000,000		$ 40,000,000
Senior Management Security Plan
Defined Benefit Plan Disclosure
Fair value of plan assets						0		0		$ 0
Executive Deferred Compensation Plan Assets						146,200,000		134,200,000
Defined Benefit Plan, Net Periodic Benefit Cost
Net periodic benefit cost						6,723,000		9,590,000		8,871,000
Amortization of net loss						(570,000)		(4,229,000)		(4,205,000)
Amortization of prior service cost						$ 219,000		$ 279,000		$ 296,000
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate						5.50%		3.00%		2.70%
Employer contributions						$ 0		$ 0
Postretirement Benefits
Defined Benefit Plan Disclosure
Fair value of plan assets						$ 31,804,000	[2]	$ 28,565,000	[2]	$ 41,464,000
Postretirement Benefit, Medical Trend Rate						6.70%		5.80%		6.30%
Postretirement Benefit, Dental Trend Rate						3.50%		3.50%		3.50%
Defined Benefit Plan, Net Periodic Benefit Cost
Net periodic benefit cost						$ 2,416,000		$ 1,096,000		$ 5,510,000
Amortization of net loss						1,237,000		31,000		0
Amortization of prior service cost						$ 1,665,000		$ 295,000		$ 47,000
Health Care Cost Trend Rate Assumed for Next Fiscal Year						3.50%		3.50%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate						5.45%		2.95%		2.70%
Employer contributions	[3]					$ 690,000		$ 1,934,000
Forecast | Pension Plan
Defined Benefit Plan, Net Periodic Benefit Cost
Defined Benefit Plan, Minimum Funding Requirement for Next Fiscal Year					$ 0
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year					$ 30,000,000
Forecast | Postretirement Benefits
Defined Benefit Plan Disclosure
Postretirement Benefit, Dental Trend Rate				3.50%
Defined Benefit Plan, Net Periodic Benefit Cost
Health Care Cost Trend Rate Assumed For Year Two					7.10%
Health Care Cost Trend Rate Assumed for Year Three				6.50%
Health Care Cost Trend Rate Assumed for Year Four		3.80%	5.80%

[1] The 2021 pension and postretirement non-service costs includes $4.7 million of expense for a temporary deviation from the cost-sharing provisions of the substantive postretirement plan as described in Note 11 - "Benefit Plans."

[2] The postretirement benefits assets are primarily life insurance contracts.

[3] Contributions and benefits paid are each net of $2.6 million and $2.9 million of plan participant contributions for 2023 and 2022, respectively.