BENEFIT PLANS: Defined Benefit Plan, Estimated Future Benefit Payments (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Pension Plan
Defined Benefit Plan Disclosure
Defined Benefit Plan, Expected Future Benefit Payments in Year One		$ 40,727
Defined Benefit Plan, Expected Future Benefit Payments in Year Two		42,674
Defined Benefit Plan, Expected Future Benefit Payments in Year Three		44,576
Defined Benefit Plan, Expected Future Benefit Payments in Year Four		46,670
Defined Benefit Plan, Expected Future Benefit Payments in Year Five		48,694
Defined Benefit Plan, Expected Future Benefit Payments in Five Fiscal Years Thereafter		273,700
Senior Management Security Plan
Defined Benefit Plan Disclosure
Defined Benefit Plan, Expected Future Benefit Payments in Year One		6,010
Defined Benefit Plan, Expected Future Benefit Payments in Year Two		6,186
Defined Benefit Plan, Expected Future Benefit Payments in Year Three		6,281
Defined Benefit Plan, Expected Future Benefit Payments in Year Four		6,700
Defined Benefit Plan, Expected Future Benefit Payments in Year Five		6,724
Defined Benefit Plan, Expected Future Benefit Payments in Five Fiscal Years Thereafter		$ 33,304
Forecast [Member] | Pension Plan
Defined Benefit Plan Disclosure
Defined Benefit Plan, Minimum Funding Requirement for Next Fiscal Year	$ 14,000