BENEFIT PLANS: Defined Benefit Plan, Assumptions Used in Calculations, Net Periodic Benefit Cost (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plan
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	4.55%	3.95%	4.45%
Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	7.50%	7.50%	7.50%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase	4.37%	4.25%	4.17%
Senior Management Security Plan
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	4.60%	3.95%	4.45%
Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	0.00%	0.00%	0.00%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase	4.75%	4.75%	4.75%
Postretirement Benefits
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	4.60%	3.95%	4.45%
Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	6.75%	6.75%	6.75%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase	0.00%	0.00%	0.00%
Postretirement Benefit, Medical Trend Rate	6.70%	6.30%	6.80%
Postretirement Benefit, Dental Trend Rate	4.00%	4.00%	4.00%