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Employee Benefit Plans: Amounts Recognized in the Consolidated Balance Sheets (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]    
Regulatory assets $ 278,258 $ 271,344
Benefit plan liabilities 150,556 154,472
Regulatory liabilities 532,720 536,652
Pension Plan    
Defined Benefit Plan Disclosure [Line Items]    
Regulatory assets 86,677 88,471
Current liabilities 0 0
Benefit plan liabilities 40,287 51,093
Regulatory liabilities 3,607 3,524
Supplemental Employee Retirement Plan    
Defined Benefit Plan Disclosure [Line Items]    
Regulatory assets 0 0
Current liabilities 1,927 1,420
Benefit plan liabilities 53,127 51,243
Regulatory liabilities 0 0
Other Postretirement Benefits Plan    
Defined Benefit Plan Disclosure [Line Items]    
Regulatory assets 16,102 11,670
Current liabilities 4,931 4,802
Benefit plan liabilities 57,142 52,136
Regulatory liabilities $ 2,140 $ 4,088