SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2023 USD ($)	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Sep. 30, 2022 USD ($)	Jun. 30, 2022 USD ($)	Mar. 31, 2022 USD ($)	Sep. 30, 2023 USD ($) segment	Sep. 30, 2022 USD ($)
Segment information
Number of reportable segments | segment							6
Operating revenues	$ 1,957.4			$ 2,003.0			$ 6,675.5	$ 7,039.0
Other operation and maintenance	516.6			454.3			1,546.6	1,357.7
Depreciation and amortization	320.3			280.3			939.7	838.0
Equity in earnings of transmission affiliates	44.7			63.7			132.1	148.4
Interest expense	182.5			127.5			533.4	364.9
Income tax expense (benefit)	60.4			73.4			183.0	263.9
Net income (loss)	315.6			301.7			1,113.2	1,157.5
Net income (loss) attributed to common shareholders	316.0	$ 289.7	$ 507.5	302.0	$ 287.5	$ 565.9	1,113.2	1,155.4
External revenues
Segment information
Operating revenues	1,957.4			2,003.0			6,675.5	7,039.0
Intersegment revenues
Segment information
Operating revenues	0.0			0.0			0.0	0.0
Utility operations
Segment information
Other operation and maintenance	495.3			450.3			1,493.4	1,341.0
Depreciation and amortization	285.8			257.4			841.4	766.6
Equity in earnings of transmission affiliates	0.0			0.0			0.0	0.0
Interest expense	174.4			158.6			526.4	472.7
Income tax expense (benefit)	76.2			67.6			261.1	286.9
Net income (loss)	262.1			204.0			885.6	845.1
Net income (loss) attributed to common shareholders	261.8			203.7			884.7	844.2
Utility operations | External revenues
Segment information
Operating revenues	1,912.9			1,976.3			6,538.8	6,941.3
Utility operations | Intersegment revenues
Segment information
Operating revenues	0.0			0.0			0.0	0.0
Reconciling eliminations
Segment information
Other operation and maintenance	(1.5)			(1.6)			(7.0)	(7.1)
Depreciation and amortization	(19.6)			(17.3)			(57.2)	(50.4)
Equity in earnings of transmission affiliates	0.0			0.0			0.0	0.0
Interest expense	(88.0)			(83.9)			(261.3)	(252.1)
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	$ (115.3)			(113.3)			$ (358.4)	(345.7)
ATC
Segment information
Ownership interest (as a percent)	60.00%						60.00%
Equity in earnings of transmission affiliates	$ 44.2			63.2			$ 130.2	146.8
ATC Holdco
Segment information
Ownership interest (as a percent)	75.00%						75.00%
Equity in earnings of transmission affiliates	$ 0.5			0.5			$ 1.9	1.6
Wisconsin | Operating Segments
Segment information
Operating revenues	1,622.0			1,676.8			5,042.8	5,176.5
Wisconsin | Operating Segments | Utility operations
Segment information
Other operation and maintenance	387.1			329.1			1,119.7	979.6
Depreciation and amortization	215.3			189.2			632.9	564.0
Equity in earnings of transmission affiliates	0.0			0.0			0.0	0.0
Interest expense	148.7			137.2			449.4	409.1
Income tax expense (benefit)	69.3			63.7			188.8	208.4
Net income (loss)	243.4			195.2			686.8	632.3
Net income (loss) attributed to common shareholders	243.1			194.9			685.9	631.4
Wisconsin | Operating Segments | Utility operations | External revenues
Segment information
Operating revenues	1,622.0			1,676.8			5,042.8	5,176.5
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	243.3			230.3			1,116.5	1,354.8
Illinois | Operating Segments | Utility operations
Segment information
Other operation and maintenance	86.5			100.2			305.5	292.9
Depreciation and amortization	59.3			57.9			176.3	172.1
Equity in earnings of transmission affiliates	0.0			0.0			0.0	0.0
Interest expense	22.0			18.1			65.0	53.8
Income tax expense (benefit)	8.9			5.8			61.8	69.1
Net income (loss)	24.7			14.9			167.9	184.7
Net income (loss) attributed to common shareholders	24.7			14.9			167.9	184.7
Illinois | Operating Segments | Utility operations | External revenues
Segment information
Operating revenues	243.3			230.3			1,116.5	1,354.8
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	47.6			69.2			379.5	410.0
Other States | Operating Segments | Utility operations
Segment information
Other operation and maintenance	21.7			21.0			68.2	68.5
Depreciation and amortization	11.2			10.3			32.2	30.5
Equity in earnings of transmission affiliates	0.0			0.0			0.0	0.0
Interest expense	3.7			3.3			12.0	9.8
Income tax expense (benefit)	(2.0)			(1.9)			10.5	9.4
Net income (loss)	(6.0)			(6.1)			30.9	28.1
Net income (loss) attributed to common shareholders	(6.0)			(6.1)			30.9	28.1
Other States | Operating Segments | Utility operations | External revenues
Segment information
Operating revenues	47.6			69.2			379.5	410.0
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	44.7			63.7			132.1	148.4
Interest expense	5.0			4.9			14.6	14.6
Income tax expense (benefit)	10.0			14.3			29.4	32.5
Net income (loss)	29.7			44.5			88.1	101.3
Net income (loss) attributed to common shareholders	29.7			44.5			88.1	101.3
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	$ 159.8			139.9			$ 495.0	443.0
Other operation and maintenance	21.5			12.1			59.6	36.9
Depreciation and amortization	48.8			34.0			139.9	102.3
Equity in earnings of transmission affiliates	0.0			0.0			0.0	0.0
Interest expense	24.8			17.0			69.8	51.6
Income tax expense (benefit)	(6.7)			1.7			(44.2)	(10.5)
Net income (loss)	66.7			70.4			240.9	244.0
Net income (loss) attributed to common shareholders	67.4			71.0			241.8	242.8
Non-Utility Energy Infrastructure | Operating Segments | External revenues
Segment information
Operating revenues	44.5			26.6			136.6	97.3
Non-Utility Energy Infrastructure | Operating Segments | Intersegment revenues
Segment information
Operating revenues	115.3			113.3			358.4	345.7
Corporate and Other | Operating Segments
Segment information
Operating revenues	0.0			0.1			0.1	0.4
Other operation and maintenance	1.3			(6.5)			0.6	(13.1)
Depreciation and amortization	5.3			6.2			15.6	19.5
Equity in earnings of transmission affiliates	0.0			0.0			0.0	0.0
Interest expense	66.3			30.9			183.9	78.1
Income tax expense (benefit)	(19.1)			(10.2)			(63.3)	(45.0)
Net income (loss)	(42.9)			(17.2)			(101.4)	(32.9)
Net income (loss) attributed to common shareholders	(42.9)			(17.2)			(101.4)	(32.9)
Corporate and Other | Operating Segments | External revenues
Segment information
Operating revenues	0.0			0.1			0.1	0.4
Corporate and Other | Operating Segments | Intersegment revenues
Segment information
Operating revenues	$ 0.0			$ 0.0			$ 0.0	$ 0.0