Revenue: Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2024	Mar. 31, 2023
Revenue from contracts with customers	$ 717.2	$ 912.1
Revenue	726.4	921.2
Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	625.7	810.4
Transportation
Revenue from contracts with customers	51.6	52.7
Wholesale
Revenue from contracts with customers	8.5	9.4
Market - off-system sales
Revenue from contracts with customers	6.6	16.4
Transmission/Other
Revenue from contracts with customers	24.8	23.2
Other revenues
Revenue	9.2	9.1
Services transferred at a point in time
Revenue from contracts with customers	8.7	8.7
Services transferred over time
Revenue from contracts with customers	708.5	903.4
Inter-segment Eliminations
Revenue from contracts with customers	(4.5)	(4.4)
Revenue	(4.5)	(4.4)
Inter-segment Eliminations \| Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	0.0	0.0
Inter-segment Eliminations \| Transportation
Revenue from contracts with customers	(0.1)	(0.1)
Inter-segment Eliminations \| Wholesale
Revenue from contracts with customers	0.0	0.0
Inter-segment Eliminations \| Market - off-system sales
Revenue from contracts with customers	0.0	0.0
Inter-segment Eliminations \| Transmission/Other
Revenue from contracts with customers	(4.4)	(4.3)
Inter-segment Eliminations \| Other revenues
Revenue	0.0	0.0
Inter-segment Eliminations \| Services transferred at a point in time
Revenue from contracts with customers	0.0	0.0
Inter-segment Eliminations \| Services transferred over time
Revenue from contracts with customers	(4.5)	(4.4)
Operating Segments \| Electric Utilities
Revenue from contracts with customers	221.4	217.8
Revenue	222.2	218.7
Operating Segments \| Electric Utilities \| Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	188.3	174.9
Operating Segments \| Electric Utilities \| Transportation
Revenue from contracts with customers	0.0	0.0
Operating Segments \| Electric Utilities \| Wholesale
Revenue from contracts with customers	8.5	9.4
Operating Segments \| Electric Utilities \| Market - off-system sales
Revenue from contracts with customers	6.6	16.1
Operating Segments \| Electric Utilities \| Transmission/Other
Revenue from contracts with customers	18.0	17.4
Operating Segments \| Electric Utilities \| Other revenues
Revenue	0.8	0.9
Operating Segments \| Electric Utilities \| Services transferred at a point in time
Revenue from contracts with customers	8.7	8.7
Operating Segments \| Electric Utilities \| Services transferred over time
Revenue from contracts with customers	212.7	209.1
Operating Segments \| Gas Utilities
Revenue from contracts with customers	500.3	698.7
Revenue	508.7	706.9
Operating Segments \| Gas Utilities \| Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	437.4	635.5
Operating Segments \| Gas Utilities \| Transportation
Revenue from contracts with customers	51.7	52.8
Operating Segments \| Gas Utilities \| Wholesale
Revenue from contracts with customers	0.0	0.0
Operating Segments \| Gas Utilities \| Market - off-system sales
Revenue from contracts with customers	0.0	0.3
Operating Segments \| Gas Utilities \| Transmission/Other
Revenue from contracts with customers	11.2	10.1
Operating Segments \| Gas Utilities \| Other revenues
Revenue	8.4	8.2
Operating Segments \| Gas Utilities \| Services transferred at a point in time
Revenue from contracts with customers	0.0	0.0
Operating Segments \| Gas Utilities \| Services transferred over time
Revenue from contracts with customers	$ 500.3	$ 698.7