Revenue: Disaggregation of Revenue (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 2,286.2	$ 2,098.0	$ 2,301.7
Revenue	2,310.0	2,127.7	2,331.3
Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,885.3	1,721.2	1,914.9
Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	194.0	177.8	176.3
Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	21.7	26.8	34.2
Market - off-system sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	52.1	34.9	51.3
Transmission
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	45.8	52.9	47.8
Other revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	87.3	84.4	77.2
Alternative revenue and other
Disaggregation of Revenue [Line Items]
Revenue	23.8	29.7	29.6
Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	34.9	34.7	31.5
Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,251.3	2,063.3	2,270.2
Inter-segment Eliminations
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(15.6)	(17.8)	(17.9)
Revenue	(15.6)	(17.8)	(17.9)
Inter-segment Eliminations \| Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0	0.0
Inter-segment Eliminations \| Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(0.4)	(0.4)	(0.5)
Inter-segment Eliminations \| Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0	0.0
Inter-segment Eliminations \| Market - off-system sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0	0.0
Inter-segment Eliminations \| Transmission
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0	0.0
Inter-segment Eliminations \| Other revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(15.2)	(17.4)	(17.4)
Inter-segment Eliminations \| Alternative revenue and other
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0	0.0
Inter-segment Eliminations \| Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0	0.0
Inter-segment Eliminations \| Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(15.6)	(17.8)	(17.9)
Operating Segments \| Electric Utilities
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	938.5	871.1	854.2
Revenue	942.8	876.1	865.0
Operating Segments \| Electric Utilities \| Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	760.4	698.6	666.1
Operating Segments \| Electric Utilities \| Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0	0.0
Operating Segments \| Electric Utilities \| Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	21.7	26.8	34.2
Operating Segments \| Electric Utilities \| Market - off-system sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	51.9	34.8	50.9
Operating Segments \| Electric Utilities \| Transmission
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	45.2	52.2	47.1
Operating Segments \| Electric Utilities \| Other revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	59.3	58.7	55.9
Operating Segments \| Electric Utilities \| Alternative revenue and other
Disaggregation of Revenue [Line Items]
Revenue	4.3	5.0	10.8
Operating Segments \| Electric Utilities \| Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	34.9	34.7	31.5
Operating Segments \| Electric Utilities \| Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	903.6	836.4	822.7
Operating Segments \| Gas Utilities
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,363.3	1,244.7	1,465.4
Revenue	1,382.8	1,269.4	1,484.2
Operating Segments \| Gas Utilities \| Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,124.9	1,022.6	1,248.8
Operating Segments \| Gas Utilities \| Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	194.4	178.2	176.8
Operating Segments \| Gas Utilities \| Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0	0.0
Operating Segments \| Gas Utilities \| Market - off-system sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.2	0.1	0.4
Operating Segments \| Gas Utilities \| Transmission
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.6	0.7	0.7
Operating Segments \| Gas Utilities \| Other revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	43.2	43.1	38.7
Operating Segments \| Gas Utilities \| Alternative revenue and other
Disaggregation of Revenue [Line Items]
Revenue	19.5	24.7	18.8
Operating Segments \| Gas Utilities \| Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0	0.0
Operating Segments \| Gas Utilities \| Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 1,363.3	$ 1,244.7	$ 1,465.4