Employee Benefit Plans: Changes in Benefit Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Pension Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of year	$ 71,178	$ 60,223
Service cost	797	704	$ 852
Interest cost	2,956	2,991	2,969
Actuarial loss (gain)	(5,650)	11,879
Benefits Paid	(3,284)	(4,452)
Asset transfer (to) from affiliate	(38)	(167)
Medicare Part D adjustment	0	0
Plan participants’ contributions	0	0
Projected benefit obligation at end of year	65,959	71,178	60,223
Supplemental Non-qualified Defined Benefit Retirement Plans [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of year	3,599	3,131
Service cost	0	0	0
Interest cost	142	146	133
Actuarial loss (gain)	(104)	540
Benefits Paid	(211)	(218)
Asset transfer (to) from affiliate	0	0
Medicare Part D adjustment	0	0
Plan participants’ contributions	0	0
Projected benefit obligation at end of year	3,426	3,599	3,131
Non-pension Defined Benefit Postretirement Healthcare Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of year	6,038	5,850
Service cost	233	222	216
Interest cost	214	241	239
Actuarial loss (gain)	27	115
Benefits Paid	(387)	(488)
Asset transfer (to) from affiliate	(7)	24
Medicare Part D adjustment	(30)	(15)
Plan participants’ contributions	120	89
Projected benefit obligation at end of year	$ 6,208	$ 6,038	$ 5,850