BENEFIT PLANS: Information about Plan Assets, Fair Value of Plan Assets by Measurement (Details) - USD ($) $ in Thousands		Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Pension Plan
Defined Benefit Plan Disclosure
Fair value of plan assets		$ 559,616		$ 559,719		$ 545,092
Pension Plan | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets		258,267		243,433
Pension Plan | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets		224,998		245,172
Pension Plan | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets		76,351		71,114		61,728
Pension Plan | Cash and Cash Equivalents
Defined Benefit Plan Disclosure
Fair value of plan assets		10,519		19,190
Pension Plan | Cash and Cash Equivalents | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets		10,519		19,190
Pension Plan | Cash and Cash Equivalents | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Pension Plan | Cash and Cash Equivalents | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Pension Plan | Short-term bonds
Defined Benefit Plan Disclosure
Fair value of plan assets		11,023		10,991
Pension Plan | Short-term bonds | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets		11,023		0
Pension Plan | Short-term bonds | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets		0		10,991
Pension Plan | Short-term bonds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Pension Plan | Intermediate bonds
Defined Benefit Plan Disclosure
Fair value of plan assets		104,241		101,867
Pension Plan | Intermediate bonds | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets		11,499		0
Pension Plan | Intermediate bonds | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets		92,742		101,867
Pension Plan | Intermediate bonds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Pension Plan | Long-term bonds
Defined Benefit Plan Disclosure
Fair value of plan assets		21,747		21,615
Pension Plan | Long-term bonds | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Pension Plan | Long-term bonds | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets		21,747		21,615
Pension Plan | Long-term bonds | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Pension Plan | Equity Securities: Large-Cap
Defined Benefit Plan Disclosure
Fair value of plan assets		73,489		66,151
Pension Plan | Equity Securities: Large-Cap | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets		73,489		66,151
Pension Plan | Equity Securities: Large-Cap | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Pension Plan | Equity Securities: Large-Cap | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Pension Plan | Equity Securities: Mid-Cap
Defined Benefit Plan Disclosure
Fair value of plan assets		64,397		68,974
Pension Plan | Equity Securities: Mid-Cap | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets		64,397		68,974
Pension Plan | Equity Securities: Mid-Cap | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Pension Plan | Equity Securities: Mid-Cap | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Pension Plan | Equity Securities: Small-Cap
Defined Benefit Plan Disclosure
Fair value of plan assets		47,777		50,972
Pension Plan | Equity Securities: Small-Cap | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets		47,777		50,972
Pension Plan | Equity Securities: Small-Cap | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Pension Plan | Equity Securities: Small-Cap | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Pension Plan | Equity Securities: Micro-Cap
Defined Benefit Plan Disclosure
Fair value of plan assets		22,186		22,962
Pension Plan | Equity Securities: Micro-Cap | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets		22,186		22,962
Pension Plan | Equity Securities: Micro-Cap | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Pension Plan | Equity Securities: Micro-Cap | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Pension Plan | Equity Securities: International
Defined Benefit Plan Disclosure
Fair value of plan assets		67,485		64,260
Pension Plan | Equity Securities: International | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets		7,698		6,555
Pension Plan | Equity Securities: International | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets		59,787		57,705
Pension Plan | Equity Securities: International | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Pension Plan | Equity Securities: Emerging Markets
Defined Benefit Plan Disclosure
Fair value of plan assets		32,846		31,544
Pension Plan | Equity Securities: Emerging Markets | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets		9,679		8,629
Pension Plan | Equity Securities: Emerging Markets | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets		23,167		22,915
Pension Plan | Equity Securities: Emerging Markets | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Pension Plan | Real Estate
Defined Benefit Plan Disclosure
Fair value of plan assets		39,035		33,996
Pension Plan | Real Estate | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Pension Plan | Real Estate | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Pension Plan | Real Estate | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets		39,035		33,996		28,019
Pension Plan | Private market investments
Defined Benefit Plan Disclosure
Fair value of plan assets		37,316		37,118
Pension Plan | Private market investments | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Pension Plan | Private market investments | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Pension Plan | Private market investments | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets		37,316		37,118		33,709
Pension Plan | Commodities
Defined Benefit Plan Disclosure
Fair value of plan assets		27,555		30,079
Pension Plan | Commodities | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Pension Plan | Commodities | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets		27,555		30,079
Pension Plan | Commodities | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Postretirement Benefits
Defined Benefit Plan Disclosure
Fair value of plan assets		35,566	[1]	38,375	[1]	$ 37,111
Postretirement Benefits | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value of plan assets	[1]	16		11
Postretirement Benefits | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value of plan assets	[1]	35,550		38,364
Postretirement Benefits | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure
Fair value of plan assets	[1]	$ 0		$ 0

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