RETIREMENT PLANS: (Details 4)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Percentage of plan assets (percent)	100.00%	100.00%
Employee Stock Ownership Plan
Defined Benefit Plan Disclosure [Line Items]
Percentage of plan assets (percent)	100.00%	100.00%
Other Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Percentage of plan assets (percent)	6.00%	4.00%
Other Securities [Member] | Employee Stock Ownership Plan
Defined Benefit Plan Disclosure [Line Items]
Percentage of plan assets (percent)	1.00%	2.00%
Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Percentage of plan assets (percent)	63.00%	63.00%
Equity Securities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Percentage of plan assets (percent)	25.00%
Equity Securities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Percentage of plan assets (percent)	75.00%
Equity Securities | Employee Stock Ownership Plan
Defined Benefit Plan Disclosure [Line Items]
Percentage of plan assets (percent)	99.00%	98.00%
Equity Securities | Employee Stock Ownership Plan | Minimum
Defined Benefit Plan Disclosure [Line Items]
Percentage of plan assets (percent)	95.00%
Equity Securities | Employee Stock Ownership Plan | Maximum
Defined Benefit Plan Disclosure [Line Items]
Percentage of plan assets (percent)	99.00%
Debt securities
Defined Benefit Plan Disclosure [Line Items]
Percentage of plan assets (percent)	31.00%	33.00%
Debt securities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Percentage of plan assets (percent)	50.00%
Debt securities | Employee Stock Ownership Plan
Defined Benefit Plan Disclosure [Line Items]
Percentage of plan assets (percent)	0.00%	0.00%
Other Security Investments [Member] | Maximum
Defined Benefit Plan Disclosure [Line Items]
Percentage of plan assets (percent)	20.00%
Other Security Investments [Member] | Employee Stock Ownership Plan | Maximum
Defined Benefit Plan Disclosure [Line Items]
Percentage of plan assets (percent)	5.00%