Revenue: Disaggregation of Revenue (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Revenue from contracts with customers	$ 2,098.0	$ 2,301.7	$ 2,542.9
Revenue	2,127.7	2,331.3	2,551.8
Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	1,721.2	1,914.9	2,162.5
Transportation
Revenue from contracts with customers	177.8	176.3	172.9
Wholesale
Revenue from contracts with customers	26.8	34.2	44.8
Market - off-system sales
Revenue from contracts with customers	34.9	51.3	49.4
Transmission
Revenue from contracts with customers	52.9	47.8	41.3
Other revenues
Revenue from contracts with customers	84.4	77.2	72.0
Alternative revenue and other
Revenue	29.7	29.6	8.9
Services transferred at a point in time
Revenue from contracts with customers	34.7	31.5	30.4
Services transferred over time
Revenue from contracts with customers	2,063.3	2,270.2	2,512.5
Inter-segment Eliminations
Revenue from contracts with customers	(17.8)	(17.9)	(17.0)
Revenue	(17.8)	(17.9)	(17.5)
Inter-segment Eliminations \| Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	0.0	0.0	0.0
Inter-segment Eliminations \| Transportation
Revenue from contracts with customers	(0.4)	(0.5)	(0.4)
Inter-segment Eliminations \| Wholesale
Revenue from contracts with customers	0.0	0.0	0.0
Inter-segment Eliminations \| Market - off-system sales
Revenue from contracts with customers	0.0	0.0	0.0
Inter-segment Eliminations \| Transmission
Revenue from contracts with customers	0.0	0.0	0.0
Inter-segment Eliminations \| Other revenues
Revenue from contracts with customers	(17.4)	(17.4)	(16.6)
Inter-segment Eliminations \| Alternative revenue and other
Revenue	0.0	0.0	(0.5)
Inter-segment Eliminations \| Services transferred at a point in time
Revenue from contracts with customers	0.0	0.0	0.0
Inter-segment Eliminations \| Services transferred over time
Revenue from contracts with customers	(17.8)	(17.9)	(17.0)
Operating Segments \| Electric Utilities
Revenue from contracts with customers	871.1	854.2	894.6
Revenue	876.1	865.0	900.2
Operating Segments \| Electric Utilities \| Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	698.6	666.1	709.2
Operating Segments \| Electric Utilities \| Transportation
Revenue from contracts with customers	0.0	0.0	0.0
Operating Segments \| Electric Utilities \| Wholesale
Revenue from contracts with customers	26.8	34.2	44.8
Operating Segments \| Electric Utilities \| Market - off-system sales
Revenue from contracts with customers	34.8	50.9	48.6
Operating Segments \| Electric Utilities \| Transmission
Revenue from contracts with customers	52.2	47.1	40.5
Operating Segments \| Electric Utilities \| Other revenues
Revenue from contracts with customers	58.7	55.9	51.5
Operating Segments \| Electric Utilities \| Alternative revenue and other
Revenue	5.0	10.8	5.6
Operating Segments \| Electric Utilities \| Services transferred at a point in time
Revenue from contracts with customers	34.7	31.5	30.4
Operating Segments \| Electric Utilities \| Services transferred over time
Revenue from contracts with customers	836.4	822.7	864.2
Operating Segments \| Gas Utilities
Revenue from contracts with customers	1,244.7	1,465.4	1,665.3
Revenue	1,269.4	1,484.2	1,669.1
Operating Segments \| Gas Utilities \| Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	1,022.6	1,248.8	1,453.3
Operating Segments \| Gas Utilities \| Transportation
Revenue from contracts with customers	178.2	176.8	173.3
Operating Segments \| Gas Utilities \| Wholesale
Revenue from contracts with customers	0.0	0.0	0.0
Operating Segments \| Gas Utilities \| Market - off-system sales
Revenue from contracts with customers	0.1	0.4	0.8
Operating Segments \| Gas Utilities \| Transmission
Revenue from contracts with customers	0.7	0.7	0.8
Operating Segments \| Gas Utilities \| Other revenues
Revenue from contracts with customers	43.1	38.7	37.1
Operating Segments \| Gas Utilities \| Alternative revenue and other
Revenue	24.7	18.8	3.8
Operating Segments \| Gas Utilities \| Services transferred at a point in time
Revenue from contracts with customers	0.0	0.0	0.0
Operating Segments \| Gas Utilities \| Services transferred over time
Revenue from contracts with customers	$ 1,244.7	$ 1,465.4	$ 1,665.3