BENEFIT PLANS: Defined Benefit Plan, Assumptions Used in Calculations, Net Periodic Benefit Cost (Details)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Plan
Defined Benefit Plan Disclosure
Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	4.25%	5.20%	4.20%
Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	7.50%	7.75%	7.75%
Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase	4.11%	4.30%	4.38%
Senior Management Security Plan
Defined Benefit Plan Disclosure
Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	4.20%	5.10%	4.15%
Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase	4.50%	4.50%	4.50%
Postretirement Benefits
Defined Benefit Plan Disclosure
Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	4.20%	5.15%	4.20%
Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	7.25%	7.25%	7.25%
Medical trend rate | Postretirement Benefits
Defined Benefit Plan Disclosure
Health Care Cost Trend Rate Assumed for Next Fiscal Year	9.70%	6.40%	6.80%
Dental trend rate | Postretirement Benefits
Defined Benefit Plan Disclosure
Health Care Cost Trend Rate Assumed for Next Fiscal Year	5.00%	5.00%	5.00%