SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2024 USD ($)	Jun. 30, 2024 USD ($)	Mar. 31, 2024 USD ($)	Sep. 30, 2023 USD ($)	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Sep. 30, 2024 USD ($) numberOfSegments	Sep. 30, 2023 USD ($)
Segment information
Number of reportable segments | numberOfSegments							6
Operating revenues	$ 1,863.5			$ 1,957.4			$ 6,315.7	$ 6,675.5
Other operation and maintenance	566.8			516.6			1,631.0	1,546.6
Depreciation and amortization	340.5			320.3			1,010.5	939.7
Equity in earnings of transmission affiliates	46.7			44.7			138.3	132.1
Interest expense	204.2			182.5			596.8	533.4
Income tax expense (benefit)	31.6			60.4			160.9	183.0
Net income (loss)	238.6			315.6			1,071.2	1,113.2
Net income (loss) attributed to common shareholders	240.1	$ 211.3	$ 622.3	316.0	$ 289.7	$ 507.5	1,073.7	1,113.2
External revenues
Segment information
Operating revenues	1,863.5			1,957.4			6,315.7	6,675.5
Intersegment revenues
Segment information
Operating revenues	0.0			0.0			0.0	0.0
Utility operations
Segment information
Other operation and maintenance	552.6			495.3			1,586.5	1,493.4
Depreciation and amortization	308.3			285.8			911.3	841.4
Equity in earnings of transmission affiliates	0.0			0.0			0.0	0.0
Interest expense	186.7			174.4			558.3	526.4
Income tax expense (benefit)	43.0			76.2			247.9	261.1
Net income (loss)	185.8			262.1			837.3	885.6
Net income (loss) attributed to common shareholders	185.5			261.8			836.4	884.7
Utility operations | External revenues
Segment information
Operating revenues	1,816.6			1,912.9			6,162.0	6,538.8
Utility operations | Intersegment revenues
Segment information
Operating revenues	0.0			0.0			0.0	0.0
Reconciling eliminations
Segment information
Other operation and maintenance	(1.6)			(1.5)			(7.1)	(7.0)
Depreciation and amortization	(22.6)			(19.6)			(65.3)	(57.2)
Equity in earnings of transmission affiliates	0.0			0.0			0.0	0.0
Interest expense	(89.7)			(88.0)			(269.7)	(261.3)
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	$ (116.2)			(115.3)			$ (355.9)	(358.4)
ATC
Segment information
Ownership interest (as a percent)	60.00%						60.00%
Equity in earnings of transmission affiliates	$ 46.1			44.2			$ 136.7	130.2
ATC Holdco
Segment information
Ownership interest (as a percent)	75.00%						75.00%
Equity in earnings of transmission affiliates	$ 0.6			0.5			$ 1.6	1.9
Wisconsin | Operating Segments
Segment information
Operating revenues	1,590.0			1,622.0			4,737.0	5,042.8
Wisconsin | Operating Segments | Utility operations
Segment information
Other operation and maintenance	403.5			387.1			1,182.6	1,119.7
Depreciation and amortization	232.4			215.3			685.3	632.9
Equity in earnings of transmission affiliates	0.0			0.0			0.0	0.0
Interest expense	160.3			148.7			475.4	449.4
Income tax expense (benefit)	63.3			69.3			172.6	188.8
Net income (loss)	238.1			243.4			637.2	686.8
Net income (loss) attributed to common shareholders	237.8			243.1			636.3	685.9
Wisconsin | Operating Segments | Utility operations | External revenues
Segment information
Operating revenues	1,590.0			1,622.0			4,737.0	5,042.8
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	173.6			243.3			1,116.4	1,116.5
Illinois | Operating Segments | Utility operations
Segment information
Other operation and maintenance	126.5			86.5			336.1	305.5
Depreciation and amortization	63.9			59.3			191.1	176.3
Equity in earnings of transmission affiliates	0.0			0.0			0.0	0.0
Interest expense	22.3			22.0			70.8	65.0
Income tax expense (benefit)	(19.0)			8.9			63.3	61.8
Net income (loss)	(48.6)			24.7			164.6	167.9
Net income (loss) attributed to common shareholders	(48.6)			24.7			164.6	167.9
Illinois | Operating Segments | Utility operations | External revenues
Segment information
Operating revenues	173.6			243.3			1,116.4	1,116.5
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	53.0			47.6			308.6	379.5
Other States | Operating Segments | Utility operations
Segment information
Other operation and maintenance	22.6			21.7			67.8	68.2
Depreciation and amortization	12.0			11.2			34.9	32.2
Equity in earnings of transmission affiliates	0.0			0.0			0.0	0.0
Interest expense	4.1			3.7			12.1	12.0
Income tax expense (benefit)	(1.3)			(2.0)			12.0	10.5
Net income (loss)	(3.7)			(6.0)			35.5	30.9
Net income (loss) attributed to common shareholders	(3.7)			(6.0)			35.5	30.9
Other States | Operating Segments | Utility operations | External revenues
Segment information
Operating revenues	53.0			47.6			308.6	379.5
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	46.7			44.7			138.3	132.1
Interest expense	4.8			5.0			14.5	14.6
Income tax expense (benefit)	10.2			10.0			30.6	29.4
Net income (loss)	31.7			29.7			93.2	88.1
Net income (loss) attributed to common shareholders	31.7			29.7			93.2	88.1
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	$ 163.1			159.8			$ 509.6	495.0
Other operation and maintenance	19.1			21.5			62.3	59.6
Depreciation and amortization	49.2			48.8			147.9	139.9
Equity in earnings of transmission affiliates	0.0			0.0			0.0	0.0
Interest expense	23.4			24.8			71.6	69.8
Income tax expense (benefit)	(16.0)			(6.7)			(59.6)	(44.2)
Net income (loss)	83.2			66.7			269.2	240.9
Net income (loss) attributed to common shareholders	85.0			67.4			272.6	241.8
Non-Utility Energy Infrastructure | Operating Segments | External revenues
Segment information
Operating revenues	46.9			44.5			153.7	136.6
Non-Utility Energy Infrastructure | Operating Segments | Intersegment revenues
Segment information
Operating revenues	116.2			115.3			355.9	358.4
Corporate and Other | Operating Segments
Segment information
Operating revenues	0.0			0.0			0.0	0.1
Other operation and maintenance	(3.3)			1.3			(10.7)	0.6
Depreciation and amortization	5.6			5.3			16.6	15.6
Equity in earnings of transmission affiliates	0.0			0.0			0.0	0.0
Interest expense	79.0			66.3			222.1	183.9
Income tax expense (benefit)	(5.6)			(19.1)			(58.0)	(63.3)
Net income (loss)	(62.1)			(42.9)			(128.5)	(101.4)
Net income (loss) attributed to common shareholders	(62.1)			(42.9)			(128.5)	(101.4)
Corporate and Other | Operating Segments | External revenues
Segment information
Operating revenues	0.0			0.0			0.0	0.1
Corporate and Other | Operating Segments | Intersegment revenues
Segment information
Operating revenues	$ 0.0			$ 0.0			$ 0.0	$ 0.0