BENEFIT PLANS: Defined Benefit Plan, Net Periodic Benefit Cost, Other Postretirement Plan (Details) - Postretirement Benefits - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan, Net Periodic Benefit Cost
Service cost	$ 1,235	$ 1,011	$ 1,315
Interest cost	2,678	2,841	2,633
Expected return on plan assets	(2,680)	(2,595)	(2,328)
Amortization of net loss	0	0	98
Amortization of prior service cost	15	183	(229)
Net periodic benefit cost	$ 1,248	$ 1,440	$ 1,489