BENEFIT PLANS: Schedule of Defined Benefit Plan Disclosures, Other Postretirment Benefit Plans (Details) - Postretirement Benefits - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1		$ 66,453		$ 70,051
Service cost		853		1,051		$ 973
Interest cost		2,989		2,643		2,783
Actuarial loss (gain)		5,298		(2,688)
Defined Benefit Plan, Benefit Obligation, Benefits Paid	[1]	(4,564)		(4,604)
Defined Benefit Plan, Accumulated Benefit Obligation, Increase (Decrease) for Plan Amendment				0
Projected benefit obligation at December 31		71,029		66,453		70,051
Change in Fair Value of Plan Assets [Roll Forward]
Fair value at January 1		33,391	[2]	38,294
Actual return (loss) on plan assets		7,269		(1,330)
Employer contributions	[1]	(3,529)		(1,031)
Benefits Paid	[1]	(4,564)		(4,604)
Fair value at December 31		39,625	[2]	33,391	[2]	$ 38,294
Funded status at end of year		(31,404)		(33,062)
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant		$ 3,300		$ 3,100

[1] Contributions and benefits paid are each net of $3.3 million and $3.1 million of plan participant contributions for 2019 and 2018, respectively.

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