BENEFIT PLANS: Benefit Plans Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan, Net Periodic Benefit Cost
Fair Value, All Levels Transfers, Amount		$ 0	$ 0
Defined Contribution Plan, Matching Contributions		7,000,000	7,000,000	$ 7,000,000
Pension Plan
Defined Benefit Plan, Net Periodic Benefit Cost
Net periodic benefit cost		40,173,000	22,676,000	44,897,000
Amortization of net loss		(13,927,000)	(3,911,000)	(17,118,000)
Amortization of prior service cost		221,000	347,000	347,000
Defined Benefit Plan, Minimum Funding Requirement for Next Fiscal Year		$ 0
Period over which bond allocations are to cover benefit payments		5 years
Period over which current rate-of-return is lower than historical		20 years
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount			23,100,000
Fair Value, All Levels Transfers, Amount		$ 0
Senior Management Security Plan
Defined Benefit Plan Disclosure
Executive Deferred Compensation Plan Assets		69,300,000	65,000,000
Defined Benefit Plan, Net Periodic Benefit Cost
Net periodic benefit cost		9,937,000	8,339,000	8,488,000
Amortization of net loss		(4,195,000)	(2,618,000)	(2,840,000)
Amortization of prior service cost		185,000	220,000	212,000
Postretirement Benefits
Defined Benefit Plan, Net Periodic Benefit Cost
Net periodic benefit cost		1,248,000	1,440,000	1,489,000
Amortization of net loss		0	0	(98,000)
Amortization of prior service cost		15,000	$ 183,000	$ (229,000)
Effect of One Percentage Point Increase on Service and Interest Cost Components		407,000
Effect of One Percentage Point Decrease on Service and Interest Cost Components		(297,000)
Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation		3,719,000
Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation		$ (2,838,000)
Postretirement Benefits | Medical trend rate
Defined Benefit Plan, Net Periodic Benefit Cost
Health Care Cost Trend Rate Assumed for Next Fiscal Year		9.70%	6.40%	6.80%
Health Care Cost Trend Rate Assumed For Year Two		8.30%
Health Care Cost Trend Rate Assumed for Year Three		6.80%
Health Care Cost Trend Rate Assumed for Year Four		5.40%
Ultimate Health Care Cost Trend Rate		4.80%
Defined Benefit Plan, Year that Rate Reaches Ultimate Trend Rate		2099
Postretirement Benefits | Dental trend rate
Defined Benefit Plan, Net Periodic Benefit Cost
Health Care Cost Trend Rate Assumed for Next Fiscal Year		5.00%	5.00%	5.00%
Post-employment Benefits
Defined Benefit Plan, Net Periodic Benefit Cost
Post employment Benefits Liability		$ 2,000,000	$ 2,000,000
Forecasted | Pension and SMS Plan
Defined Benefit Plan, Net Periodic Benefit Cost
Net periodic benefit cost	$ 17,300,000
Forecasted | Pension Plan
Defined Benefit Plan, Net Periodic Benefit Cost
Amortization of net loss	13,500,000
Amortization of prior service cost	100,000
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year	20,000,000
Forecasted | Senior Management Security Plan
Defined Benefit Plan, Net Periodic Benefit Cost
Amortization of net loss	3,500,000
Amortization of prior service cost	200,000
Forecasted | Postretirement Benefits
Defined Benefit Plan, Net Periodic Benefit Cost
Net periodic benefit cost	26,000
Amortization of prior service cost	$ 26,000