BENEFIT PLANS: Schedule of Defined Benefit Plan Disclosures, Other Postretirment Benefit Plans (Details) - Postretirement Benefits - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning balance		$ 65,999		$ 57,341
Service cost		1,235		1,011		$ 1,315
Interest cost		2,678		2,841		2,633
Actuarial (gain) loss		(5,008)		7,026
Benefits paid	[1]	(2,511)		(2,220)
Benefit obligation, ending balance		62,393		65,999		57,341
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning balance		38,375	[2]	37,111
Actual return on plan assets		85		3,888
Employer contributions	[1]	(383)		(404)
Benefits paid	[1]	(2,511)		(2,220)
Fair value of plan assets, ending balance		35,566	[2]	38,375	[2]	$ 37,111
Funded status at end of year		(26,827)		(27,624)
Plan participant contributions		3,518		3,379
Medicare Part D subsidy receipts		$ 330		$ 344

[1] Contributions and benefits paid are each net of $3,518 thousand and $3,379 thousand of plan participant contributions, and $330 thousand and $344 thousand of Medicare Part D subsidy receipts for 2015 and 2014, respectively.

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