Employee Benefits - Change in Benefit Obligations and Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Pension Benefits
Change in benefit obligation
Obligation at January 1	$ 2,315.9	$ 3,136.6
Service cost	24.0	50.8	$ 54.3
Interest cost	122.3	91.8	87.5
Participant contributions	0.0	0.0
Plan amendments	0.0	0.0
Actuarial (gain) loss	81.9	(682.3)
Benefit payments	(191.7)	(281.0)
Transfer	0.0	0.0
Obligation at December 31	2,352.4	2,315.9	3,136.6
Change in fair value of plan assets
Beginning balance at January 1	2,628.0	3,328.9
Actual return on plan assets	214.9	(431.3)
Employer contributions net of plan transfer	14.6	11.4
Participant contributions	0.0	0.0
Benefit payments	(191.7)	(281.0)
Ending balance at December 31	2,665.8	2,628.0	3,328.9
Funded status at December 31	313.4	312.1
OPEB Benefits
Change in benefit obligation
Obligation at January 1	402.3	530.2
Service cost	9.8	14.3	15.7
Interest cost	21.6	15.4	14.5
Participant contributions	11.8	12.5
Plan amendments	0.0	0.2
Actuarial (gain) loss	45.9	(127.9)
Benefit payments	(46.0)	(45.7)
Federal subsidy on benefits paid	1.5	1.4
Transfer	1.2	1.9
Obligation at December 31	448.1	402.3	530.2
Change in fair value of plan assets
Beginning balance at January 1	835.3	1,000.2
Actual return on plan assets	76.4	(135.4)
Employer contributions net of plan transfer	(47.9)	3.7
Participant contributions	11.8	12.5
Benefit payments	(46.0)	(45.7)
Ending balance at December 31	829.6	835.3	$ 1,000.2
Funded status at December 31	381.5	$ 433.0
Amount transferred out of WEC Energy Group Retiree Welfare Plan	$ 50.0