BENEFIT PLANS: Schedule of Defined Benefit Plan Disclosures, Other Postretirment Benefit Plans (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Change in Fair Value of Plan Assets [Roll Forward]
Employer contributions		$ (7,900)		$ (7,700)		$ (7,700)
Postretirement Benefits
Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1		71,029		66,453
Service cost		1,029		853		1,051
Interest cost		2,493		2,989		2,643
Actuarial loss (gain)		9,359		5,298
Defined Benefit Plan, Benefit Obligation, Benefits Paid	[1]	(2,958)		(4,564)
Projected benefit obligation at December 31		80,952		71,029		66,453
Change in Fair Value of Plan Assets [Roll Forward]
Fair value at January 1		39,625	[2]	33,391
Actual return (loss) on plan assets		5,248		7,269
Employer contributions	[1]	604		(3,529)
Benefits Paid	[1]	(2,958)		(4,564)
Fair value at December 31		41,311	[2]	39,625	[2]	$ 33,391
Funded status at end of year		(39,641)		(31,404)
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant		$ 3,400		$ 3,300

[1] Contributions and benefits paid are each net of $3.4 million and $3.3 million of plan participant contributions for 2020 and 2019, respectively.

[2] The postretirement benefits assets are primarily life insurance contracts.