Revenue: Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Revenue from contracts with customers	$ 435.7	$ 397.1	$ 1,236.1	$ 1,114.3
Revenue	439.0	402.6	1,244.2	1,129.0
Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	346.6	316.3	1,037.6	933.3
Transportation
Revenue from contracts with customers	42.0	35.8	99.6	87.4
Wholesale
Revenue from contracts with customers	4.2	6.5	11.3	15.0
Market - off-system sales
Revenue from contracts with customers	10.8	5.5	22.1	12.0
Transmission
Revenue from contracts with customers	10.1	13.0	22.5	26.0
Other revenues
Revenue from contracts with customers	22.0	20.0	43.0	40.6
Alternative revenue and other
Revenue	3.3	5.5	8.1	14.7
Services transferred at a point in time
Revenue from contracts with customers	9.1	7.8	17.2	16.5
Services transferred over time
Revenue from contracts with customers	426.6	389.3	1,218.9	1,097.8
Inter-segment Eliminations
Revenue from contracts with customers	(3.9)	(4.5)	(7.8)	(8.9)
Revenue	(3.9)	(4.5)	(7.8)	(8.9)
Inter-segment Eliminations \| Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	0.0	0.0	0.0	0.0
Inter-segment Eliminations \| Transportation
Revenue from contracts with customers	(0.1)	(0.2)	(0.2)	(0.2)
Inter-segment Eliminations \| Wholesale
Revenue from contracts with customers	0.0	0.0	0.0	0.0
Inter-segment Eliminations \| Market - off-system sales
Revenue from contracts with customers	0.0	0.0	0.0	0.0
Inter-segment Eliminations \| Transmission
Revenue from contracts with customers	0.0	0.0	0.0	0.0
Inter-segment Eliminations \| Other revenues
Revenue from contracts with customers	(3.8)	(4.3)	(7.6)	(8.7)
Inter-segment Eliminations \| Alternative revenue and other
Revenue	0.0	0.0	0.0	0.0
Inter-segment Eliminations \| Services transferred at a point in time
Revenue from contracts with customers	0.0	0.0	0.0	0.0
Inter-segment Eliminations \| Services transferred over time
Revenue from contracts with customers	(3.9)	(4.5)	(7.8)	(8.9)
Operating Segments \| Electric Utilities
Revenue from contracts with customers	218.6	202.7	454.2	424.1
Revenue	219.9	205.1	456.6	427.3
Operating Segments \| Electric Utilities \| Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	178.1	163.4	369.3	343.0
Operating Segments \| Electric Utilities \| Transportation
Revenue from contracts with customers	0.0	0.0	0.0	0.0
Operating Segments \| Electric Utilities \| Wholesale
Revenue from contracts with customers	4.2	6.5	11.3	15.0
Operating Segments \| Electric Utilities \| Market - off-system sales
Revenue from contracts with customers	10.7	5.5	22.0	12.0
Operating Segments \| Electric Utilities \| Transmission
Revenue from contracts with customers	10.0	12.9	22.2	25.6
Operating Segments \| Electric Utilities \| Other revenues
Revenue from contracts with customers	15.6	14.4	29.4	28.5
Operating Segments \| Electric Utilities \| Alternative revenue and other
Revenue	1.3	2.4	2.4	3.2
Operating Segments \| Electric Utilities \| Services transferred at a point in time
Revenue from contracts with customers	9.1	7.8	17.2	16.5
Operating Segments \| Electric Utilities \| Services transferred over time
Revenue from contracts with customers	209.5	194.9	437.0	407.6
Operating Segments \| Gas Utilities
Revenue from contracts with customers	221.0	198.9	789.7	699.1
Revenue	223.0	202.0	795.4	710.6
Operating Segments \| Gas Utilities \| Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	168.5	152.9	668.3	590.3
Operating Segments \| Gas Utilities \| Transportation
Revenue from contracts with customers	42.1	36.0	99.8	87.6
Operating Segments \| Gas Utilities \| Wholesale
Revenue from contracts with customers	0.0	0.0	0.0	0.0
Operating Segments \| Gas Utilities \| Market - off-system sales
Revenue from contracts with customers	0.1	0.0	0.1	0.0
Operating Segments \| Gas Utilities \| Transmission
Revenue from contracts with customers	0.1	0.1	0.3	0.4
Operating Segments \| Gas Utilities \| Other revenues
Revenue from contracts with customers	10.2	9.9	21.2	20.8
Operating Segments \| Gas Utilities \| Alternative revenue and other
Revenue	2.0	3.1	5.7	11.5
Operating Segments \| Gas Utilities \| Services transferred at a point in time
Revenue from contracts with customers	0.0	0.0	0.0	0.0
Operating Segments \| Gas Utilities \| Services transferred over time
Revenue from contracts with customers	$ 221.0	$ 198.9	$ 789.7	$ 699.1