SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Jun. 30, 2022 USD ($)	Mar. 31, 2022 USD ($)	Jun. 30, 2023 USD ($) segment	Jun. 30, 2022 USD ($)
Segment information
Number of reportable segments | segment					6
Operating revenues	$ 1,830.0		$ 2,127.9		$ 4,718.1	$ 5,036.0
Other operation and maintenance	496.0		449.0		1,030.0	903.4
Depreciation and amortization	313.9		279.6		619.4	557.7
Equity in earnings of transmission affiliates	43.6		43.0		87.4	84.7
Interest expense	178.7		119.8		350.9	237.4
Income tax expense (benefit)	48.5		63.4		122.6	190.5
Net income (loss)	290.0		287.8		797.6	855.8
Net income (loss) attributed to common shareholders	289.7	$ 507.5	287.5	$ 565.9	797.2	853.4
External revenues
Segment information
Operating revenues	1,830.0		2,127.9		4,718.1	5,036.0
Intersegment revenues
Segment information
Operating revenues	0.0		0.0		0.0	0.0
Utility operations
Segment information
Other operation and maintenance	478.9		439.9		998.1	890.7
Depreciation and amortization	279.4		255.3		555.6	509.2
Equity in earnings of transmission affiliates	0.0		0.0		0.0	0.0
Interest expense	175.6		156.8		352.0	314.1
Income tax expense (benefit)	65.8		71.4		184.9	219.3
Net income (loss)	219.7		207.8		623.5	641.1
Net income (loss) attributed to common shareholders	219.4		207.5		622.9	640.5
Utility operations | External revenues
Segment information
Operating revenues	1,779.9		2,099.7		4,625.9	4,965.0
Utility operations | Intersegment revenues
Segment information
Operating revenues	0.0		0.0		0.0	0.0
Reconciling eliminations
Segment information
Other operation and maintenance	(3.9)		(3.9)		(5.5)	(5.5)
Depreciation and amortization	(19.1)		(16.8)		(37.6)	(33.1)
Equity in earnings of transmission affiliates	0.0		0.0		0.0	0.0
Interest expense	(88.8)		(83.8)		(173.3)	(168.2)
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	$ (119.0)		(115.5)		$ (243.1)	(232.4)
ATC
Segment information
Ownership interest (as a percent)	60.00%				60.00%
Equity in earnings of transmission affiliates	$ 43.1		42.6		$ 86.0	83.6
ATC Holdco
Segment information
Ownership interest (as a percent)	75.00%				75.00%
Equity in earnings of transmission affiliates	$ 0.5		0.4		$ 1.4	1.1
Wisconsin
Segment information
Operating revenues	1,424.5		1,557.4		3,420.8	3,499.7
Wisconsin | Utility operations
Segment information
Other operation and maintenance	351.8		337.9		732.6	650.5
Depreciation and amortization	210.3		187.7		417.6	374.8
Equity in earnings of transmission affiliates	0.0		0.0		0.0	0.0
Interest expense	150.1		135.6		300.7	271.9
Income tax expense (benefit)	53.6		49.3		119.5	144.7
Net income (loss)	185.9		148.7		443.4	437.1
Net income (loss) attributed to common shareholders	185.6		148.4		442.8	436.5
Wisconsin | Utility operations | External revenues
Segment information
Operating revenues	1,424.5		1,557.4		3,420.8	3,499.7
Wisconsin | Utility operations | Intersegment revenues
Segment information
Operating revenues	0.0		0.0		0.0	0.0
Illinois
Segment information
Operating revenues	273.5		442.4		873.2	1,124.5
Illinois | Utility operations
Segment information
Other operation and maintenance	105.3		79.1		219.0	192.7
Depreciation and amortization	58.5		57.4		117.0	114.2
Equity in earnings of transmission affiliates	0.0		0.0		0.0	0.0
Interest expense	21.4		18.0		43.0	35.7
Income tax expense (benefit)	10.9		21.2		52.9	63.3
Net income (loss)	30.1		56.4		143.2	169.8
Net income (loss) attributed to common shareholders	30.1		56.4		143.2	169.8
Illinois | Utility operations | External revenues
Segment information
Operating revenues	273.5		442.4		873.2	1,124.5
Illinois | Utility operations | Intersegment revenues
Segment information
Operating revenues	0.0		0.0		0.0	0.0
Other States
Segment information
Operating revenues	81.9		99.9		331.9	340.8
Other States | Utility operations
Segment information
Other operation and maintenance	21.8		22.9		46.5	47.5
Depreciation and amortization	10.6		10.2		21.0	20.2
Equity in earnings of transmission affiliates	0.0		0.0		0.0	0.0
Interest expense	4.1		3.2		8.3	6.5
Income tax expense (benefit)	1.3		0.9		12.5	11.3
Net income (loss)	3.7		2.7		36.9	34.2
Net income (loss) attributed to common shareholders	3.7		2.7		36.9	34.2
Other States | Utility operations | External revenues
Segment information
Operating revenues	81.9		99.9		331.9	340.8
Other States | Utility operations | Intersegment revenues
Segment information
Operating revenues	0.0		0.0		0.0	0.0
Electric Transmission
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	43.6		43.0		87.4	84.7
Interest expense	4.8		4.8		9.6	9.7
Income tax expense (benefit)	9.7		9.3		19.4	18.2
Net income (loss)	29.1		29.0		58.4	56.8
Net income (loss) attributed to common shareholders	29.1		29.0		58.4	56.8
Electric Transmission | External revenues
Segment information
Operating revenues	0.0		0.0		0.0	0.0
Electric Transmission | 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%
Operating revenues	$ 169.0		143.6		$ 335.2	303.1
Other operation and maintenance	20.3		13.9		38.1	24.8
Depreciation and amortization	48.4		34.3		91.1	68.3
Equity in earnings of transmission affiliates	0.0		0.0		0.0	0.0
Interest expense	25.1		17.4		45.0	34.6
Income tax expense (benefit)	(19.7)		(7.3)		(37.5)	(12.2)
Net income (loss)	85.9		80.3		174.2	173.6
Net income (loss) attributed to common shareholders	85.9		80.3		174.4	171.8
Non-Utility Energy Infrastructure | External revenues
Segment information
Operating revenues	50.0		28.1		92.1	70.7
Non-Utility Energy Infrastructure | Intersegment revenues
Segment information
Operating revenues	119.0		115.5		243.1	232.4
Corporate and Other
Segment information
Operating revenues	0.1		0.1		0.1	0.3
Other operation and maintenance	0.7		(0.9)		(0.7)	(6.6)
Depreciation and amortization	5.2		6.8		10.3	13.3
Equity in earnings of transmission affiliates	0.0		0.0		0.0	0.0
Interest expense	62.0		24.6		117.6	47.2
Income tax expense (benefit)	(7.3)		(10.0)		(44.2)	(34.8)
Net income (loss)	(44.7)		(29.3)		(58.5)	(15.7)
Net income (loss) attributed to common shareholders	(44.7)		(29.3)		(58.5)	(15.7)
Corporate and Other | External revenues
Segment information
Operating revenues	0.1		0.1		0.1	0.3
Corporate and Other | Intersegment revenues
Segment information
Operating revenues	$ 0.0		$ 0.0		$ 0.0	$ 0.0