SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2024 USD ($)	Mar. 31, 2024 USD ($)	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Jun. 30, 2024 USD ($) numberOfSegments	Jun. 30, 2023 USD ($)
Segment information
Number of reportable segments | numberOfSegments					6
Operating revenues	$ 1,772.0		$ 1,830.0		$ 4,452.2	$ 4,718.1
Other operation and maintenance	533.4		496.0		1,064.2	1,030.0
Depreciation and amortization	336.6		313.9		670.0	619.4
Equity in earnings of transmission affiliates	46.8		43.6		91.6	87.4
Interest expense	200.6		178.7		392.6	350.9
Income tax expense (benefit)	41.6		48.5		129.3	122.6
Net income (loss)	210.0		290.0		832.6	797.6
Net income (loss) attributed to common shareholders	211.3	$ 622.3	289.7	$ 507.5	833.6	797.2
External revenues
Segment information
Operating revenues	1,772.0		1,830.0		4,452.2	4,718.1
Intersegment revenues
Segment information
Operating revenues	0.0		0.0		0.0	0.0
Utility operations
Segment information
Other operation and maintenance	516.4		478.9		1,033.9	998.1
Depreciation and amortization	303.5		279.4		603.0	555.6
Equity in earnings of transmission affiliates	0.0		0.0		0.0	0.0
Interest expense	184.8		175.6		371.6	352.0
Income tax expense (benefit)	44.9		65.8		204.9	184.9
Net income (loss)	158.7		219.7		651.5	623.5
Net income (loss) attributed to common shareholders	158.4		219.4		650.9	622.9
Utility operations | External revenues
Segment information
Operating revenues	1,716.0		1,779.9		4,345.4	4,625.9
Utility operations | Intersegment revenues
Segment information
Operating revenues	0.0		0.0		0.0	0.0
Reconciling eliminations
Segment information
Other operation and maintenance	(4.0)		(3.9)		(5.5)	(5.5)
Depreciation and amortization	(21.9)		(19.1)		(42.7)	(37.6)
Equity in earnings of transmission affiliates	0.0		0.0		0.0	0.0
Interest expense	(89.7)		(88.8)		(180.0)	(173.3)
Income tax expense (benefit)	0.0		0.0		0.0	0.0
Net income (loss)	0.0		0.0		0.0	0.0
Net income (loss) attributed to common shareholders	0.0		0.0		0.0	0.0
Reconciling eliminations | External revenues
Segment information
Operating revenues	0.0		0.0		0.0	0.0
Reconciling eliminations | Intersegment revenues
Segment information
Operating revenues	$ (119.6)		(119.0)		$ (239.7)	(243.1)
ATC
Segment information
Ownership interest (as a percent)	60.00%				60.00%
Equity in earnings of transmission affiliates	$ 46.2		43.1		$ 90.6	86.0
ATC Holdco
Segment information
Ownership interest (as a percent)	75.00%				75.00%
Equity in earnings of transmission affiliates	$ 0.6		0.5		$ 1.0	1.4
Wisconsin | Operating Segments
Segment information
Operating revenues	1,368.2		1,424.5		3,147.0	3,420.8
Wisconsin | Operating Segments | Utility operations
Segment information
Other operation and maintenance	389.2		351.8		779.1	732.6
Depreciation and amortization	228.3		210.3		452.9	417.6
Equity in earnings of transmission affiliates	0.0		0.0		0.0	0.0
Interest expense	157.3		150.1		315.1	300.7
Income tax expense (benefit)	34.4		53.6		109.3	119.5
Net income (loss)	132.4		185.9		399.1	443.4
Net income (loss) attributed to common shareholders	132.1		185.6		398.5	442.8
Wisconsin | Operating Segments | Utility operations | External revenues
Segment information
Operating revenues	1,368.2		1,424.5		3,147.0	3,420.8
Wisconsin | Operating Segments | Utility operations | Intersegment revenues
Segment information
Operating revenues	0.0		0.0		0.0	0.0
Illinois | Operating Segments
Segment information
Operating revenues	276.8		273.5		942.8	873.2
Illinois | Operating Segments | Utility operations
Segment information
Other operation and maintenance	102.6		105.3		209.6	219.0
Depreciation and amortization	63.7		58.5		127.2	117.0
Equity in earnings of transmission affiliates	0.0		0.0		0.0	0.0
Interest expense	23.5		21.4		48.5	43.0
Income tax expense (benefit)	10.2		10.9		82.3	52.9
Net income (loss)	25.7		30.1		213.2	143.2
Net income (loss) attributed to common shareholders	25.7		30.1		213.2	143.2
Illinois | Operating Segments | Utility operations | External revenues
Segment information
Operating revenues	276.8		273.5		942.8	873.2
Illinois | Operating Segments | Utility operations | Intersegment revenues
Segment information
Operating revenues	0.0		0.0		0.0	0.0
Other States | Operating Segments
Segment information
Operating revenues	71.0		81.9		255.6	331.9
Other States | Operating Segments | Utility operations
Segment information
Other operation and maintenance	24.6		21.8		45.2	46.5
Depreciation and amortization	11.5		10.6		22.9	21.0
Equity in earnings of transmission affiliates	0.0		0.0		0.0	0.0
Interest expense	4.0		4.1		8.0	8.3
Income tax expense (benefit)	0.3		1.3		13.3	12.5
Net income (loss)	0.6		3.7		39.2	36.9
Net income (loss) attributed to common shareholders	0.6		3.7		39.2	36.9
Other States | Operating Segments | Utility operations | External revenues
Segment information
Operating revenues	71.0		81.9		255.6	331.9
Other States | Operating Segments | Utility operations | Intersegment revenues
Segment information
Operating revenues	0.0		0.0		0.0	0.0
Electric Transmission | Operating Segments
Segment information
Other operation and maintenance	0.0		0.0		0.0	0.0
Depreciation and amortization	0.0		0.0		0.0	0.0
Equity in earnings of transmission affiliates	46.8		43.6		91.6	87.4
Interest expense	4.9		4.8		9.7	9.6
Income tax expense (benefit)	10.5		9.7		20.4	19.4
Net income (loss)	31.4		29.1		61.5	58.4
Net income (loss) attributed to common shareholders	31.4		29.1		61.5	58.4
Electric Transmission | Operating Segments | External revenues
Segment information
Operating revenues	0.0		0.0		0.0	0.0
Electric Transmission | Operating Segments | Intersegment revenues
Segment information
Operating revenues	$ 0.0		0.0		$ 0.0	0.0
Electric Transmission | ATC
Segment information
Ownership interest (as a percent)	60.00%				60.00%
Electric Transmission | ATC Holdco
Segment information
Ownership interest (as a percent)	75.00%				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	$ 175.6		169.0		$ 346.5	335.2
Other operation and maintenance	25.0		20.3		43.2	38.1
Depreciation and amortization	49.6		48.4		98.7	91.1
Equity in earnings of transmission affiliates	0.0		0.0		0.0	0.0
Interest expense	24.1		25.1		48.2	45.0
Income tax expense (benefit)	(20.2)		(19.7)		(43.6)	(37.5)
Net income (loss)	91.7		85.9		186.0	174.2
Net income (loss) attributed to common shareholders	93.3		85.9		187.6	174.4
Non-Utility Energy Infrastructure | Operating Segments | External revenues
Segment information
Operating revenues	56.0		50.0		106.8	92.1
Non-Utility Energy Infrastructure | Operating Segments | Intersegment revenues
Segment information
Operating revenues	119.6		119.0		239.7	243.1
Corporate and Other | Operating Segments
Segment information
Operating revenues	0.0		0.1		0.0	0.1
Other operation and maintenance	(4.0)		0.7		(7.4)	(0.7)
Depreciation and amortization	5.4		5.2		11.0	10.3
Equity in earnings of transmission affiliates	0.0		0.0		0.0	0.0
Interest expense	76.5		62.0		143.1	117.6
Income tax expense (benefit)	6.4		(7.3)		(52.4)	(44.2)
Net income (loss)	(71.8)		(44.7)		(66.4)	(58.5)
Net income (loss) attributed to common shareholders	(71.8)		(44.7)		(66.4)	(58.5)
Corporate and Other | Operating Segments | External revenues
Segment information
Operating revenues	0.0		0.1		0.0	0.1
Corporate and Other | Operating Segments | Intersegment revenues
Segment information
Operating revenues	$ 0.0		$ 0.0		$ 0.0	$ 0.0