BENEFIT PLANS: Defined Benefit Plan, Assumptions Used in Calculations, Benefit Obligations (Details)	12 Months Ended
	Dec. 31, 2015		Dec. 31, 2014	Dec. 31, 2013
Pension Plan
Defined Benefit Plan Disclosure
Assumptions Used Calculating Benefit Obligation, Discount Rate	4.60%		4.25%
Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase	4.11%	[1]	4.30%
Measurement Date	12/31/2015		12/31/2014
Senior Management Security Plan
Defined Benefit Plan Disclosure
Assumptions Used Calculating Benefit Obligation, Discount Rate	4.60%		4.20%
Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase	4.50%		4.50%
Measurement Date	12/31/2015		12/31/2014
Postretirement Benefits
Defined Benefit Plan Disclosure
Assumptions Used Calculating Benefit Obligation, Discount Rate	4.60%		4.20%
Measurement Date	12/31/2015		12/31/2014
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%
Inflation rate | Pension Plan
Defined Benefit Plan Disclosure
Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase	2.50%
Composite merit increase | Pension Plan
Defined Benefit Plan Disclosure
Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase	1.61%
Merit salary increase first year of service | Pension Plan
Defined Benefit Plan Disclosure
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 Disclosure
Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase	0.00%

[1] The 2015 rate of compensation increase assumption for the pension plan includes an inflation component of 2.50% plus a 1.61% 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% for employees in their fortieth year of service and beyond.