BENEFIT PLANS: Defined Benefit Plan, Assumptions Used in Calculations, Benefit Obligations (Details)		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plan
Defined Benefit Plan Disclosure
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets		7.50%	7.50%	7.50%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Assumptions Used Calculating Benefit Obligation, Discount Rate		3.60%	4.55%
Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase	[1]	4.37%	4.25%
Defined Benefit Plan, Plan Assets, Accounting Policy Election, Measurement Date		Dec. 31, 2019	Dec. 31, 2018
Senior Management Security Plan
Defined Benefit Plan Disclosure
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets		0.00%	0.00%	0.00%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Assumptions Used Calculating Benefit Obligation, Discount Rate		3.65%	4.60%
Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase	[1]	4.75%	4.75%
Defined Benefit Plan, Plan Assets, Accounting Policy Election, Measurement Date		Dec. 31, 2019	Dec. 31, 2018
Postretirement Benefits
Defined Benefit Plan Disclosure
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets		6.75%	6.75%	6.75%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Assumptions Used Calculating Benefit Obligation, Discount Rate		3.60%	4.60%
Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase	[1]	0.00%	0.00%
Postretirement Benefit, Medical Trend Rate		6.70%	6.30%	6.80%
Health Care Cost Trend Rate Assumed for Next Fiscal Year		4.00%	4.00%
Defined Benefit Plan, Plan Assets, Accounting Policy Election, Measurement Date		Dec. 31, 2019	Dec. 31, 2018
Inflation rate | Pension Plan
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase		2.40%
Composite merit increase | Pension Plan
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase		1.97%
Merit salary increase first year of service | Pension Plan
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase		8.00%
Merit salary increase fortieth year of service and beyond | Pension Plan
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase		0.60%

[1] The 2019 rate of compensation increase assumption for the pension plan includes an inflation component of 2.40% plus a 1.97% composite merit increase component that is based on employees' years of service. Merit salary increases are assumed to be 8.0% for employees in their first year of service and scale down to 0.6% for employees in their fortieth year of service and beyond.