BENEFIT PLANS: Schedule of Defined Benefit Plan Disclosures, Other Postretirment Benefit Plans (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2021		Dec. 31, 2020		Dec. 31, 2019
Change in Fair Value of Plan Assets [Roll Forward]
Employer contributions		$ (8,200)		$ (7,900)		$ (7,700)
Postretirement Benefits
Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1		80,952		71,029
Service cost		1,063		1,029		853
Interest cost		2,059		2,493		2,989
Actuarial loss (gain)		(5,805)		9,359
Defined Benefit Plan, Benefit Obligation, Benefits Paid	[1]	(4,194)		(2,958)
Projected benefit obligation at December 31		74,075		80,952		71,029
Change in Fair Value of Plan Assets [Roll Forward]
Fair value at January 1		41,311	[2]	39,625
Actual return (loss) on plan assets		6,308		5,248
Employer contributions	[1]	(1,961)		(604)
Benefits Paid	[1]	(4,194)		(2,958)
Fair value at December 31		41,464	[2]	41,311	[2]	$ 39,625
Funded status at end of year		(32,611)		(39,641)
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant		$ 3,000		$ 3,400

[1] Contributions and benefits paid are each net of $3.0 million and $3.4 million of plan participant contributions for 2021 and 2020, respectively.

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