SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended
	Mar. 31, 2024 USD ($) numberOfSegments	Mar. 31, 2023 USD ($)
Segment information
Number of reportable segments | numberOfSegments	6
Operating revenues	$ 2,680.2	$ 2,888.1
Other operation and maintenance	530.8	534.0
Depreciation and amortization	333.4	305.5
Equity in earnings of transmission affiliates	44.8	43.8
Interest expense	192.0	172.2
Income tax expense (benefit)	87.7	74.1
Net income (loss)	622.6	507.6
Net income (loss) attributed to common shareholders	622.3	507.5
External revenues
Segment information
Operating revenues	2,680.2	2,888.1
Intersegment revenues
Segment information
Operating revenues	0.0	0.0
Utility operations
Segment information
Other operation and maintenance	517.5	519.2
Depreciation and amortization	299.5	276.2
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	186.8	176.4
Income tax expense (benefit)	160.0	119.1
Net income (loss)	492.8	403.8
Net income (loss) attributed to common shareholders	492.5	403.5
Utility operations | External revenues
Segment information
Operating revenues	2,629.4	2,846.0
Utility operations | Intersegment revenues
Segment information
Operating revenues	0.0	0.0
Reconciling eliminations
Segment information
Other operation and maintenance	(1.5)	(1.6)
Depreciation and amortization	(20.8)	(18.5)
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	(90.3)	(84.5)
Income tax expense (benefit)	0.0	0.0
Net income (loss)	0.0	0.0
Net income (loss) attributed to common shareholders	0.0	0.0
Reconciling eliminations | External revenues
Segment information
Operating revenues	0.0	0.0
Reconciling eliminations | Intersegment revenues
Segment information
Operating revenues	$ (120.1)	(124.1)
ATC
Segment information
Ownership interest (as a percent)	60.00%
Equity in earnings of transmission affiliates	$ 44.4	42.9
ATC Holdco
Segment information
Ownership interest (as a percent)	75.00%
Equity in earnings of transmission affiliates	$ 0.4	0.9
Wisconsin | Operating Segments
Segment information
Operating revenues	1,778.8	1,996.3
Wisconsin | Operating Segments | Utility operations
Segment information
Other operation and maintenance	389.9	380.8
Depreciation and amortization	224.6	207.3
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	157.8	150.6
Income tax expense (benefit)	74.9	65.9
Net income (loss)	266.7	257.5
Net income (loss) attributed to common shareholders	266.4	257.2
Wisconsin | Operating Segments | Utility operations | External revenues
Segment information
Operating revenues	1,778.8	1,996.3
Wisconsin | Operating Segments | Utility operations | Intersegment revenues
Segment information
Operating revenues	0.0	0.0
Illinois | Operating Segments
Segment information
Operating revenues	666.0	599.7
Illinois | Operating Segments | Utility operations
Segment information
Other operation and maintenance	107.0	113.7
Depreciation and amortization	63.5	58.5
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	25.0	21.6
Income tax expense (benefit)	72.1	42.0
Net income (loss)	187.5	113.1
Net income (loss) attributed to common shareholders	187.5	113.1
Illinois | Operating Segments | Utility operations | External revenues
Segment information
Operating revenues	666.0	599.7
Illinois | Operating Segments | Utility operations | Intersegment revenues
Segment information
Operating revenues	0.0	0.0
Other States | Operating Segments
Segment information
Operating revenues	184.6	250.0
Other States | Operating Segments | Utility operations
Segment information
Other operation and maintenance	20.6	24.7
Depreciation and amortization	11.4	10.4
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	4.0	4.2
Income tax expense (benefit)	13.0	11.2
Net income (loss)	38.6	33.2
Net income (loss) attributed to common shareholders	38.6	33.2
Other States | Operating Segments | Utility operations | External revenues
Segment information
Operating revenues	184.6	250.0
Other States | Operating Segments | Utility operations | Intersegment revenues
Segment information
Operating revenues	0.0	0.0
Electric Transmission | Operating Segments
Segment information
Other operation and maintenance	0.0	0.0
Depreciation and amortization	0.0	0.0
Equity in earnings of transmission affiliates	44.8	43.8
Interest expense	4.8	4.8
Income tax expense (benefit)	9.9	9.7
Net income (loss)	30.1	29.3
Net income (loss) attributed to common shareholders	30.1	29.3
Electric Transmission | Operating Segments | External revenues
Segment information
Operating revenues	0.0	0.0
Electric Transmission | Operating Segments | Intersegment revenues
Segment information
Operating revenues	$ 0.0	0.0
Electric Transmission | ATC
Segment information
Ownership interest (as a percent)	60.00%
Electric Transmission | ATC Holdco
Segment information
Ownership interest (as a percent)	75.00%
Non-Utility Energy Infrastructure
Segment information
Natural gas storage needs provided to Wisconsin utilities	33.00%
Non-Utility Energy Infrastructure | Operating Segments
Segment information
Operating revenues	$ 170.9	166.2
Other operation and maintenance	18.2	17.8
Depreciation and amortization	49.1	42.7
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	24.1	19.9
Income tax expense (benefit)	(23.4)	(17.8)
Net income (loss)	94.3	88.3
Net income (loss) attributed to common shareholders	94.3	88.5
Non-Utility Energy Infrastructure | Operating Segments | External revenues
Segment information
Operating revenues	50.8	42.1
Non-Utility Energy Infrastructure | Operating Segments | Intersegment revenues
Segment information
Operating revenues	120.1	124.1
Corporate and Other | Operating Segments
Segment information
Operating revenues	0.0	0.0
Other operation and maintenance	(3.4)	(1.4)
Depreciation and amortization	5.6	5.1
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	66.6	55.6
Income tax expense (benefit)	(58.8)	(36.9)
Net income (loss)	5.4	(13.8)
Net income (loss) attributed to common shareholders	5.4	(13.8)
Corporate and Other | Operating Segments | External revenues
Segment information
Operating revenues	0.0	0.0
Corporate and Other | Operating Segments | Intersegment revenues
Segment information
Operating revenues	$ 0.0	$ 0.0