Revenue: Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Revenue from contracts with customers	$ 427.4	$ 398.6	$ 1,663.6	$ 1,513.0
Revenue	430.2	401.6	1,674.5	1,530.6
Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	329.9	303.8	1,367.6	1,237.2
Transportation
Revenue from contracts with customers	43.5	43.2	143.1	130.9
Wholesale
Revenue from contracts with customers	5.5	6.1	16.9	21.1
Market - off-system sales
Revenue from contracts with customers	16.3	10.7	38.4	22.8
Transmission
Revenue from contracts with customers	11.2	13.6	33.6	39.5
Other revenues
Revenue from contracts with customers	21.0	21.2	64.0	61.5
Alternative revenue and other
Revenue	2.8	3.0	10.9	17.6
Services transferred at a point in time
Revenue from contracts with customers	8.8	9.3	25.9	25.9
Services transferred over time
Revenue from contracts with customers	418.6	389.3	1,637.7	1,487.1
Inter-segment Eliminations
Revenue from contracts with customers	(3.9)	(4.5)	(11.7)	(13.4)
Revenue	(3.9)	(4.5)	(11.7)	(13.4)
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)	(0.0)	(0.3)	(0.0)
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)	(4.5)	(11.4)	(13.4)
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)	(11.7)	(13.4)
Operating Segments \| Electric Utilities
Revenue from contracts with customers	248.8	231.6	703.0	655.7
Revenue	249.7	232.5	706.3	659.8
Operating Segments \| Electric Utilities \| Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	201.3	186.3	570.6	529.4
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	5.5	6.1	16.9	21.1
Operating Segments \| Electric Utilities \| Market - off-system sales
Revenue from contracts with customers	16.3	10.7	38.3	22.8
Operating Segments \| Electric Utilities \| Transmission
Revenue from contracts with customers	11.1	13.6	33.3	39.1
Operating Segments \| Electric Utilities \| Other revenues
Revenue from contracts with customers	14.6	14.9	43.9	43.3
Operating Segments \| Electric Utilities \| Alternative revenue and other
Revenue	0.9	0.9	3.3	4.1
Operating Segments \| Electric Utilities \| Services transferred at a point in time
Revenue from contracts with customers	8.8	9.3	25.9	25.9
Operating Segments \| Electric Utilities \| Services transferred over time
Revenue from contracts with customers	240.0	222.3	677.1	629.8
Operating Segments \| Gas Utilities
Revenue from contracts with customers	182.5	171.5	972.3	870.7
Revenue	184.4	173.6	979.9	884.2
Operating Segments \| Gas Utilities \| Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	128.6	117.5	797.0	707.8
Operating Segments \| Gas Utilities \| Transportation
Revenue from contracts with customers	43.6	43.2	143.4	130.9
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	0.0	0.0	0.1	0.0
Operating Segments \| Gas Utilities \| Transmission
Revenue from contracts with customers	0.1	0.0	0.3	0.4
Operating Segments \| Gas Utilities \| Other revenues
Revenue from contracts with customers	10.2	10.8	31.5	31.6
Operating Segments \| Gas Utilities \| Alternative revenue and other
Revenue	1.9	2.1	7.6	13.5
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	$ 182.5	$ 171.5	$ 972.3	$ 870.7