Operating Revenues - Disaggregation of Natural Gas Utility Operating Revenues by Customer Class (Details) - Revenues from contracts with customers - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 8,785.3	$ 9,568.1	$ 8,235.7
Natural gas
Disaggregation of Operating Revenues
Revenues from contracts with customers	3,582.6	4,468.7	3,617.6
Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	3,580.9	4,466.2	3,614.6
Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	3,251.5	4,222.9	3,092.1
Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	2,302.4	2,922.7	2,188.0
Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	949.1	1,300.2	904.1
Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	357.4	376.1	343.0
Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	(28.0)	(132.8)	179.5
Wisconsin | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,606.7	1,980.7	1,490.3
Wisconsin | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,518.7	1,906.7	1,401.0
Wisconsin | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,012.0	1,234.0	928.9
Wisconsin | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	506.7	672.7	472.1
Wisconsin | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	93.0	81.8	80.0
Wisconsin | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	(5.0)	(7.8)	9.3
Illinois | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,480.5	1,883.7	1,630.3
Illinois | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,233.1	1,706.2	1,320.0
Illinois | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	966.0	1,297.4	1,017.9
Illinois | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	267.1	408.8	302.1
Illinois | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	231.9	259.8	231.2
Illinois | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	15.5	(82.3)	79.1
Other States | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	493.7	601.8	494.0
Other States | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	499.7	610.0	371.1
Other States | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	324.4	391.3	241.2
Other States | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	175.3	218.7	129.9
Other States | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	32.5	34.5	31.8
Other States | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ (38.5)	$ (42.7)	$ 91.1