Property, Plant and Equipment (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Public Utility, Property, Plant and Equipment [Line Items]
Revisions to cash flow and timing estimates	$ 12,921	$ 44,679
Direct first mortgage lien	2,153,800
Non-contractually required removal costs of utility plant in service	680,900	694,000
Contractually required removal costs of utility plant in service	249,900	218,700
Regulatory assets, non-current	541,784	593,939
Regulatory assets, non-current	541,784	593,939
Entity Information [Line Items]
Regulatory assets, non-current	541,784	593,939
Contractually required removal costs of utility plant in service	249,900	218,700
Non-contractually required removal costs of utility plant in service	680,900	694,000
Direct first mortgage lien	2,153,800
Petersburg Unit 1 retirement
Public Utility, Property, Plant and Equipment [Line Items]
Regulatory assets, non-current	35,700
Regulatory assets, non-current	35,700
Entity Information [Line Items]
Regulatory assets, non-current	35,700
Petersburg Unit 1 retirement and 2 retirement costs
Public Utility, Property, Plant and Equipment [Line Items]
Regulatory assets, non-current	259,892	287,463
Regulatory assets, non-current	259,892	287,463
Entity Information [Line Items]
Regulatory assets, non-current	259,892	287,463
Subsidiaries [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Revisions to cash flow and timing estimates	12,921	44,679
Direct first mortgage lien	2,153,800
Non-contractually required removal costs of utility plant in service	680,900	694,000
Contractually required removal costs of utility plant in service	249,900	218,700
Regulatory assets, non-current	541,784	593,939
Regulatory assets, non-current	541,784	593,939
Entity Information [Line Items]
Regulatory assets, non-current	541,784	593,939
Contractually required removal costs of utility plant in service	249,900	218,700
Non-contractually required removal costs of utility plant in service	680,900	694,000
Direct first mortgage lien	2,153,800
Subsidiaries [Member] | Petersburg Unit 1 retirement
Public Utility, Property, Plant and Equipment [Line Items]
Regulatory assets, non-current		287,463
Regulatory assets, non-current		287,463
Entity Information [Line Items]
Regulatory assets, non-current		$ 287,463
Subsidiaries [Member] | Petersburg Unit 1 retirement and 2 retirement costs
Public Utility, Property, Plant and Equipment [Line Items]
Regulatory assets, non-current	259,892
Regulatory assets, non-current	259,892
Entity Information [Line Items]
Regulatory assets, non-current	$ 259,892