Revenue: Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Revenue from contracts with customers	$ 397.1	$ 405.1	$ 1,114.3	$ 1,317.1
Revenue	402.6	411.3	1,129.0	1,332.4
Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	324.2	331.2	949.9	1,141.6
Transportation
Revenue from contracts with customers	35.8	35.8	87.4	88.6
Wholesale
Revenue from contracts with customers	6.5	5.7	15.0	15.1
Market - off-system sales
Revenue from contracts with customers	5.5	8.4	12.1	24.8
Transmission/Other
Revenue from contracts with customers	25.1	24.0	49.9	47.0
Other revenues
Revenue	5.5	6.2	14.7	15.3
Services transferred at a point in time
Revenue from contracts with customers	7.8	7.8	16.5	16.5
Services transferred over time
Revenue from contracts with customers	389.3	397.3	1,097.8	1,300.6
Inter-segment Eliminations
Revenue from contracts with customers	(4.5)	(4.5)	(8.9)	(9.0)
Revenue	(4.5)	(4.5)	(8.9)	(9.0)
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)	(0.2)	(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/Other
Revenue from contracts with customers	(4.3)	(4.4)	(8.7)	(8.8)
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)	(8.9)	(9.0)
Operating Segments \| Electric Utilities
Revenue from contracts with customers	202.7	189.7	424.1	407.5
Revenue	205.1	193.1	427.3	411.8
Operating Segments \| Electric Utilities \| Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	171.3	156.4	359.6	331.3
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	6.5	5.7	15.0	15.1
Operating Segments \| Electric Utilities \| Market - off-system sales
Revenue from contracts with customers	5.5	8.4	12.0	24.5
Operating Segments \| Electric Utilities \| Transmission/Other
Revenue from contracts with customers	19.4	19.2	37.5	36.6
Operating Segments \| Electric Utilities \| Other revenues
Revenue	2.4	3.4	3.2	4.3
Operating Segments \| Electric Utilities \| Services transferred at a point in time
Revenue from contracts with customers	7.8	7.8	16.5	16.5
Operating Segments \| Electric Utilities \| Services transferred over time
Revenue from contracts with customers	194.9	181.9	407.6	391.0
Operating Segments \| Gas Utilities
Revenue from contracts with customers	198.9	219.9	699.1	918.6
Revenue	202.0	222.7	710.6	929.6
Operating Segments \| Gas Utilities \| Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	152.9	174.8	590.3	810.3
Operating Segments \| Gas Utilities \| Transportation
Revenue from contracts with customers	36.0	35.9	87.6	88.8
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	0.3
Operating Segments \| Gas Utilities \| Transmission/Other
Revenue from contracts with customers	10.0	9.2	21.1	19.2
Operating Segments \| Gas Utilities \| Other revenues
Revenue	3.1	2.8	11.5	11.0
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	$ 198.9	$ 219.9	$ 699.1	$ 918.6