Revenue: Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 440.6	$ 435.7	$ 1,200.6	$ 1,236.1
Revenue	452.8	439.0	1,233.5	1,244.2
Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	349.4	346.6	1,005.2	1,037.6
Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	44.7	42.0	99.1	99.6
Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	5.3	4.2	11.3	11.3
Market - off-system sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2.5	10.8	13.4	22.1
Transmission
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	17.7	10.1	30.0	22.5
Other revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	21.0	22.0	41.6	43.0
Alternative revenue and other
Disaggregation of Revenue [Line Items]
Revenue	12.2	3.3	32.9	8.1
Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	8.0	9.1	16.8	17.2
Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	432.6	426.6	1,183.8	1,218.9
Inter-segment Eliminations
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(4.0)	(3.9)	(8.0)	(7.8)
Revenue	(4.0)	(3.9)	(8.0)	(7.8)
Inter-segment Eliminations \| Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0	0.0	0.0
Inter-segment Eliminations \| Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(0.1)	(0.1)	(0.2)	(0.2)
Inter-segment Eliminations \| Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0	0.0	0.0
Inter-segment Eliminations \| Market - off-system sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0	0.0	0.0
Inter-segment Eliminations \| Transmission
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0	0.0	0.0
Inter-segment Eliminations \| Other revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(3.9)	(3.8)	(7.8)	(7.6)
Inter-segment Eliminations \| Alternative revenue and other
Disaggregation of Revenue [Line Items]
Revenue	0.0	0.0	0.0	0.0
Inter-segment Eliminations \| Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0	0.0	0.0
Inter-segment Eliminations \| Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(4.0)	(3.9)	(8.0)	(7.8)
Operating Segments
Disaggregation of Revenue [Line Items]
Revenue	456.8	442.9	1,241.5	1,252.0
Operating Segments \| Electric Utilities
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	224.2	218.6	464.8	454.2
Revenue	226.3	219.9	467.9	456.6
Operating Segments \| Electric Utilities \| Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	184.4	178.1	381.4	369.3
Operating Segments \| Electric Utilities \| Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0	0.0	0.0
Operating Segments \| Electric Utilities \| Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	5.3	4.2	11.3	11.3
Operating Segments \| Electric Utilities \| Market - off-system sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2.5	10.7	13.4	22.0
Operating Segments \| Electric Utilities \| Transmission
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	17.5	10.0	29.6	22.2
Operating Segments \| Electric Utilities \| Other revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	14.5	15.6	29.1	29.4
Operating Segments \| Electric Utilities \| Alternative revenue and other
Disaggregation of Revenue [Line Items]
Revenue	2.1	1.3	3.1	2.4
Operating Segments \| Electric Utilities \| Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	8.0	9.1	16.8	17.2
Operating Segments \| Electric Utilities \| Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	216.2	209.5	448.0	437.0
Operating Segments \| Gas Utilities
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	220.4	221.0	743.8	789.7
Revenue	230.5	223.0	773.6	795.4
Operating Segments \| Gas Utilities \| Retail - Electric , Natural Gas and Coal
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	165.0	168.5	623.8	668.3
Operating Segments \| Gas Utilities \| Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	44.8	42.1	99.3	99.8
Operating Segments \| Gas Utilities \| Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0	0.0	0.0
Operating Segments \| Gas Utilities \| Market - off-system sales
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.1	0.0	0.1
Operating Segments \| Gas Utilities \| Transmission
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.2	0.1	0.4	0.3
Operating Segments \| Gas Utilities \| Other revenues
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	10.4	10.2	20.3	21.2
Operating Segments \| Gas Utilities \| Alternative revenue and other
Disaggregation of Revenue [Line Items]
Revenue	10.1	2.0	29.8	5.7
Operating Segments \| Gas Utilities \| Services transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0	0.0	0.0
Operating Segments \| Gas Utilities \| Services transferred over time
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 220.4	$ 221.0	$ 743.8	$ 789.7