BENEFIT PLANS: Expected future benefit payments and prescription drug benefits (Details) - Postretirement Benefits $ in Thousands	Dec. 31, 2019 USD ($)
Defined Benefit Plan Disclosure
Defined Benefit Plan, Expected Future Benefit Payments in Year One	$ 5,552
Defined Benefit Plan, Expected Future Benefit Payments in Year Two	4,932
Defined Benefit Plan, Expected Future Benefit Payments in Year Three	4,750
Defined Benefit Plan, Expected Future Benefit Payments in Year Four	4,532
Defined Benefit Plan, Expected Future Benefit Payments in Year Five	4,289
Defined Benefit Plan, Expected Future Benefit Payments in Five Fiscal Years Thereafter	$ 19,133