Revenue: Disaggregation of Revenue (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Revenue from contracts with customers	$ 2,301.7	$ 2,542.9	$ 1,930.9
Revenue	2,331.3	2,551.8	1,949.1
Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	1,946.5	2,193.0	1,625.2
Transportation
Revenue from contracts with customers	176.3	172.9	157.7
Wholesale
Revenue from contracts with customers	34.2	44.8	30.8
Market - off-system sales
Revenue from contracts with customers	51.3	49.4	42.1
Transmission/Other
Revenue from contracts with customers	93.4	82.8	75.1
Other revenues
Revenue	29.6	8.9	18.2
Services transferred at a point in time
Revenue from contracts with customers	31.5	30.4	27.1
Services transferred over time
Revenue from contracts with customers	2,270.2	2,512.5	1,903.8
Inter-segment Eliminations
Revenue from contracts with customers	(17.9)	(17.0)	(17.6)
Revenue	(17.9)	(17.5)	(18.0)
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	(0.5)	(0.4)	(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/Other
Revenue from contracts with customers	(17.4)	(16.6)	(17.2)
Inter-segment Eliminations \| Other revenues
Revenue	0.0	(0.5)	(0.4)
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	(17.9)	(17.0)	(17.6)
Operating Segments \| Electric Utilities
Revenue from contracts with customers	854.2	894.6	836.9
Revenue	865.0	900.2	842.2
Operating Segments \| Electric Utilities \| Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	697.7	739.7	711.5
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	34.2	44.8	30.8
Operating Segments \| Electric Utilities \| Market - off-system sales
Revenue from contracts with customers	50.9	48.6	41.7
Operating Segments \| Electric Utilities \| Transmission/Other
Revenue from contracts with customers	71.4	61.5	52.9
Operating Segments \| Electric Utilities \| Other revenues
Revenue	10.8	5.6	5.3
Operating Segments \| Electric Utilities \| Services transferred at a point in time
Revenue from contracts with customers	31.5	30.4	27.1
Operating Segments \| Electric Utilities \| Services transferred over time
Revenue from contracts with customers	822.7	864.2	809.8
Operating Segments \| Gas Utilities
Revenue from contracts with customers	1,465.4	1,665.3	1,111.6
Revenue	1,484.2	1,669.1	1,124.9
Operating Segments \| Gas Utilities \| Retail - Electric , Natural Gas and Coal
Revenue from contracts with customers	1,248.8	1,453.3	913.7
Operating Segments \| Gas Utilities \| Transportation
Revenue from contracts with customers	176.8	173.3	158.1
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	0.4	0.8	0.4
Operating Segments \| Gas Utilities \| Transmission/Other
Revenue from contracts with customers	39.4	37.9	39.4
Operating Segments \| Gas Utilities \| Other revenues
Revenue	18.8	3.8	13.3
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	$ 1,465.4	$ 1,665.3	$ 1,111.6