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	$ 894.9	$ 1,144.0
Service cost	10.3	13.4	$ 14.4
Interest cost	47.1	32.6	31.1
Participant contributions	0.0	0.0
Net transfer from/to affiliates	(1.2)	4.7
Actuarial (gain) loss	29.1	(211.3)
Benefit payments	(79.4)	(88.5)
Transfer	0.0	0.0
Obligation at December 31	900.8	894.9	1,144.0
Change in fair value of plan assets
Beginning balance at January 1	940.7	1,136.9
Actual return on plan assets	68.9	(115.8)
Employer contributions net of plan transfer	3.2	3.8
Participant contributions	0.0	0.0
Net transfer from/to affiliates	(1.2)	4.3
Benefit payments	(79.4)	(88.5)
Ending balance at December 31	932.2	940.7	1,136.9
Funded status at December 31	31.4	45.8
OPEB Benefits
Change in benefit obligation
Obligation at January 1	144.6	190.6
Service cost	2.6	3.9	4.3
Interest cost	7.7	5.6	5.3
Participant contributions	5.6	6.6
Net transfer from/to affiliates	(0.4)	0.0
Actuarial (gain) loss	19.5	(46.9)
Benefit payments	(18.7)	(19.0)
Federal subsidy on benefits paid	1.3	1.2
Transfer	1.2	2.6
Obligation at December 31	163.4	144.6	190.6
Change in fair value of plan assets
Beginning balance at January 1	211.3	255.7
Actual return on plan assets	17.7	(32.6)
Employer contributions net of plan transfer	42.9	0.1
Participant contributions	5.6	6.6
Net transfer from/to affiliates	(0.2)	0.5
Benefit payments	(18.7)	(19.0)
Ending balance at December 31	172.8	211.3	$ 255.7
Funded status at December 31	9.4	$ 66.7
Amount transferred out of WEC Energy Group Retiree Welfare Plan	$ 43.0