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BENEFIT PLANS: Amounts recognized in accumulated other comprehensive income, other postretirement plan (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure    
Regulatory Assets $ 1,383,059 $ 1,214,174
Postretirement Benefits    
Defined Benefit Plan Disclosure    
Net (loss) gain (81) (330)
Prior service cost 174 222
Subtotal 93 (108)
Regulatory Assets (93) (108)
Net amount recognized in accumulated other comprehensive income $ 0 $ 0