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Employee Benefit Plans: Amounts Recognized in the Consolidated Balance Sheet (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Regulatory assets $ 284,235 $ 297,454
Non-current liabilities 145,147 159,646
Regulatory liabilities 540,794 485,126
Pension Plans, Defined Benefit    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Regulatory assets 82,919 72,756
Current liabilities 0 0
Non-current assets 0 0
Non-current liabilities 54,585 58,381
Regulatory liabilities 4,620 5,232
Supplemental Employee Retirement Plans, Defined Benefit    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Regulatory assets 0 0
Current liabilities 1,463 1,372
Non-current assets 0 0
Non-current liabilities 41,547 43,739
Regulatory liabilities 0 0
Other Postretirement Benefit Plans, Defined Benefit    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Regulatory assets 6,655 11,507
Current liabilities 3,885 4,423
Non-current assets 249 69
Non-current liabilities 49,015 56,365
Regulatory liabilities $ 5,207 $ 3,334