BENEFIT PLANS: Information about Plan Assets, Fair Value of Plan Assets by Measurement (Details) - USD ($) $ in Thousands		Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Pension Plan
Defined Benefit Plan Disclosure
Fair value of plan assets		$ 763,119		$ 650,604		$ 697,683
Pension Plan | Cash and Cash Equivalents
Defined Benefit Plan Disclosure
Fair value of plan assets		10,878		9,717
Pension Plan | Short-term bonds
Defined Benefit Plan Disclosure
Fair value of plan assets		21,628		20,644
Pension Plan | Intermediate Bonds
Defined Benefit Plan Disclosure
Fair value of plan assets		157,300		108,241
Pension Plan | Long-term bonds
Defined Benefit Plan Disclosure
Fair value of plan assets		0		40,857
Pension Plan | Equity Securities: Large Cap
Defined Benefit Plan Disclosure
Fair value of plan assets		92,852		71,176
Pension Plan | Equity Securities: Mid Cap
Defined Benefit Plan Disclosure
Fair value of plan assets		81,663		71,419
Pension Plan | Equity Securities: Small Cap
Defined Benefit Plan Disclosure
Fair value of plan assets		67,075		53,401
Pension Plan | Equity Securities: Micro-Cap
Defined Benefit Plan Disclosure
Fair value of plan assets		31,469		30,387
Pension Plan | Equity Securities: International
Defined Benefit Plan Disclosure
Fair value of plan assets		13,817		7,104
Pension Plan | Defined Benefit Plan, Equity Securities, Non-US [Member]
Defined Benefit Plan Disclosure
Fair value of plan assets		8,245		6,519
Pension Plan | Equity Securities: Emerging Markets
Defined Benefit Plan Disclosure
Fair value of plan assets		40,059		29,757
Pension Plan | Comingled Fund: Commodities Investment
Defined Benefit Plan Disclosure
Fair value of plan assets		34,793		30,842
Pension Plan | Real Estate
Defined Benefit Plan Disclosure
Fair value of plan assets		47,570		39,846
Pension Plan | Private market investments
Defined Benefit Plan Disclosure
Fair value of plan assets		40,795		35,041
Postretirement Benefits
Defined Benefit Plan Disclosure
Fair value of plan assets		39,625	[1]	33,391	[1]	$ 38,294
Fair Value, Inputs, Level 1 | Pension Plan
Defined Benefit Plan Disclosure
Fair value of plan assets		349,996		290,962
Fair Value, Inputs, Level 1 | Pension Plan | Cash and Cash Equivalents
Defined Benefit Plan Disclosure
Fair value of plan assets		10,878		9,717
Fair Value, Inputs, Level 1 | Pension Plan | Short-term bonds
Defined Benefit Plan Disclosure
Fair value of plan assets		21,628		20,644
Fair Value, Inputs, Level 1 | Pension Plan | Intermediate Bonds
Defined Benefit Plan Disclosure
Fair value of plan assets		22,369		20,595
Fair Value, Inputs, Level 1 | Pension Plan | Long-term bonds
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Fair Value, Inputs, Level 1 | Pension Plan | Equity Securities: Large Cap
Defined Benefit Plan Disclosure
Fair value of plan assets		92,852		71,176
Fair Value, Inputs, Level 1 | Pension Plan | Equity Securities: Mid Cap
Defined Benefit Plan Disclosure
Fair value of plan assets		81,663		71,419
Fair Value, Inputs, Level 1 | Pension Plan | Equity Securities: Small Cap
Defined Benefit Plan Disclosure
Fair value of plan assets		67,075		53,401
Fair Value, Inputs, Level 1 | Pension Plan | Equity Securities: Micro-Cap
Defined Benefit Plan Disclosure
Fair value of plan assets		31,469		30,387
Fair Value, Inputs, Level 1 | Pension Plan | Equity Securities: International
Defined Benefit Plan Disclosure
Fair value of plan assets		13,817		7,104
Fair Value, Inputs, Level 1 | Pension Plan | Defined Benefit Plan, Equity Securities, Non-US [Member]
Defined Benefit Plan Disclosure
Fair value of plan assets		8,245		6,519
Fair Value, Inputs, Level 1 | Postretirement Benefits
Defined Benefit Plan Disclosure
Fair value of plan assets	[1]	641		758
Fair Value, Inputs, Level 2 | Pension Plan
Defined Benefit Plan Disclosure
Fair value of plan assets		134,931		128,503
Fair Value, Inputs, Level 2 | Pension Plan | Cash and Cash Equivalents
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Fair Value, Inputs, Level 2 | Pension Plan | Short-term bonds
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Fair Value, Inputs, Level 2 | Pension Plan | Intermediate Bonds
Defined Benefit Plan Disclosure
Fair value of plan assets		134,931		87,646
Fair Value, Inputs, Level 2 | Pension Plan | Long-term bonds
Defined Benefit Plan Disclosure
Fair value of plan assets		0		40,857
Fair Value, Inputs, Level 2 | Pension Plan | Equity Securities: Large Cap
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Fair Value, Inputs, Level 2 | Pension Plan | Equity Securities: Mid Cap
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Fair Value, Inputs, Level 2 | Pension Plan | Equity Securities: Small Cap
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Fair Value, Inputs, Level 2 | Pension Plan | Equity Securities: Micro-Cap
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Fair Value, Inputs, Level 2 | Pension Plan | Equity Securities: International
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Fair Value, Inputs, Level 2 | Pension Plan | Defined Benefit Plan, Equity Securities, Non-US [Member]
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Fair Value, Inputs, Level 2 | Postretirement Benefits
Defined Benefit Plan Disclosure
Fair value of plan assets	[1]	38,984		32,633
Fair Value, Inputs, Level 3 | Pension Plan
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Fair Value, Inputs, Level 3 | Pension Plan | Cash and Cash Equivalents
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Fair Value, Inputs, Level 3 | Pension Plan | Short-term bonds
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Fair Value, Inputs, Level 3 | Pension Plan | Intermediate Bonds
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Fair Value, Inputs, Level 3 | Pension Plan | Long-term bonds
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Fair Value, Inputs, Level 3 | Pension Plan | Equity Securities: Large Cap
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Fair Value, Inputs, Level 3 | Pension Plan | Equity Securities: Mid Cap
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Fair Value, Inputs, Level 3 | Pension Plan | Equity Securities: Small Cap
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Fair Value, Inputs, Level 3 | Pension Plan | Equity Securities: Micro-Cap
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Fair Value, Inputs, Level 3 | Pension Plan | Equity Securities: International
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Fair Value, Inputs, Level 3 | Pension Plan | Defined Benefit Plan, Equity Securities, Non-US [Member]
Defined Benefit Plan Disclosure
Fair value of plan assets		0		0
Fair Value, Inputs, Level 3 | Postretirement Benefits
Defined Benefit Plan Disclosure
Fair value of plan assets	[1]	0		0
Fair Value Measured at Net Asset Value Per Share [Member] | Pension Plan | Equity Securities: International
Defined Benefit Plan Disclosure
Fair value of plan assets		$ 114,975		$ 95,653

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