EMPLOYEE BENEFIT PLANS: Postretirement (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Estimated future benefit payments
2016	$ 448
2017	415
2018	451
2019	495
2020	539
2021 through 2025	$ 3,214
Postretirement benefit plans
Postretirement benefit plan disclosure
Reduction in benefit obligation due to plan amendment			$ 10,425
Assumed ultimate health care cost trend rate (as a percent)	3.00%
Decrease in discount rate (as a percent)	0.30%
Discount rate used in determining the accumulated postretirement benefit obligation (as a percent)	4.13%
Accumulated benefit obligation after plan amendment	$ 11,400	$ 12,311
Amounts recognized in accumulated other comprehensive loss (pre-tax)
Prior service credit	(8,098)
Net actuarial loss	(2,371)
Net amount recognized in accumulated other comprehensive (gain) loss	(10,469)
Estimated amount to be amortized from accumulated other comprehensive loss (gain) into net periodic benefit cost during next fiscal year
Actuarial loss (gain)	291
Prior service credit (gain)	1,352
Changes in the accumulated postretirement benefit obligation
Benefit obligation, beginning of the period	12,311	9,176
Service cost	441	342	1,036
Interest cost	465	423	1,060
Actuarial (gain)/loss	(1,580)	2,611
Benefits paid	(237)	(241)
Benefit obligation, end of the period	11,400	12,311	9,176
Net periodic postretirement benefit cost
Service cost benefits attributed to service during the period	441	342	1,036
Interest cost on the accumulated postretirement benefit obligation	465	423	1,060
Net amortization	(1,451)	(1,804)	671
Net periodic postretirement benefit cost	$ (545)	$ (1,039)	$ 2,767