Revenue: Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Revenue from contracts with customers	$ 398.6	$ 400.1	$ 1,513.0	$ 1,717.4
Revenue	401.6	407.1	1,530.6	1,739.6
Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	312.9	311.8	1,262.8	1,453.4
Transportation
Revenue from contracts with customers	43.0	42.6	130.5	131.1
Wholesale
Revenue from contracts with customers	6.2	9.4	21.2	24.6
Market - off-system sales
Revenue from contracts with customers	10.8	13.0	22.9	37.9
Transmission/Other
Revenue from contracts with customers	25.7	23.3	75.6	70.4
Other revenues
Revenue	3.0	7.0	17.6	22.2
Services transferred at a point in time
Revenue from contracts with customers	9.3	7.8	25.9	24.3
Services transferred over time
Revenue from contracts with customers	389.3	392.3	1,487.1	1,693.1
Inter-segment Eliminations
Revenue from contracts with customers	(4.5)	(4.5)	(13.4)	(13.4)
Revenue	(4.5)	(4.5)	(13.4)	(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	(0.2)	(0.1)	(0.3)	(0.3)
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/Other
Revenue from contracts with customers	(4.3)	(4.4)	(13.1)	(13.1)
Inter-segment Eliminations \| Other revenues
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	(4.5)	(4.5)	(13.4)	(13.4)
Operating Segments \| Electric Utilities
Revenue from contracts with customers	231.6	231.5	655.7	639.1
Revenue	232.5	237.3	659.8	649.1
Operating Segments \| Electric Utilities \| Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	195.5	191.0	555.1	522.3
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	6.2	9.4	21.2	24.6
Operating Segments \| Electric Utilities \| Market - off-system sales
Revenue from contracts with customers	10.8	13.0	22.8	37.5
Operating Segments \| Electric Utilities \| Transmission/Other
Revenue from contracts with customers	19.1	18.1	56.6	54.7
Operating Segments \| Electric Utilities \| Other revenues
Revenue	0.9	5.8	4.1	10.0
Operating Segments \| Electric Utilities \| Services transferred at a point in time
Revenue from contracts with customers	9.3	7.8	25.9	24.3
Operating Segments \| Electric Utilities \| Services transferred over time
Revenue from contracts with customers	222.3	223.7	629.8	614.8
Operating Segments \| Gas Utilities
Revenue from contracts with customers	171.5	173.1	870.7	1,091.7
Revenue	173.6	174.3	884.2	1,103.9
Operating Segments \| Gas Utilities \| Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	117.4	120.8	707.7	931.1
Operating Segments \| Gas Utilities \| Transportation
Revenue from contracts with customers	43.2	42.7	130.8	131.4
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	0.4
Operating Segments \| Gas Utilities \| Transmission/Other
Revenue from contracts with customers	10.9	9.6	32.1	28.8
Operating Segments \| Gas Utilities \| Other revenues
Revenue	2.1	1.2	13.5	12.2
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	$ 171.5	$ 173.1	$ 870.7	$ 1,091.7