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, 2021	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 8,235.7	$ 7,169.3	$ 7,449.3
Natural gas
Disaggregation of Operating Revenues
Revenues from contracts with customers	3,617.6	2,826.9	3,071.4
Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	3,614.6	2,824.5	3,068.1
Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	3,092.1	2,450.5	2,784.2
Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	2,188.0	1,775.6	1,953.9
Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	904.1	674.9	830.3
Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	343.0	326.2	349.5
Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	179.5	47.8	(65.6)
Wisconsin | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	6,006.9	5,461.7	5,631.8
Wisconsin | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,490.3	1,195.6	1,324.1
Wisconsin | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,401.0	1,090.7	1,257.8
Wisconsin | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	928.9	752.6	837.9
Wisconsin | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	472.1	338.1	419.9
Wisconsin | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	80.0	79.1	72.6
Wisconsin | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	9.3	25.8	(6.3)
Illinois | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,630.3	1,267.9	1,332.5
Illinois | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,630.3	1,267.9	1,332.4
Illinois | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,320.0	1,023.2	1,119.5
Illinois | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,017.9	802.2	857.8
Illinois | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	302.1	221.0	261.7
Illinois | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	231.2	215.6	245.3
Illinois | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	79.1	29.1	(32.4)
Other States | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	511.8	378.1	428.2
Other States | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	494.0	361.0	411.6
Other States | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	371.1	336.6	406.9
Other States | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	241.2	220.8	258.2
Other States | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	129.9	115.8	148.7
Other States | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	31.8	31.5	31.6
Other States | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 91.1	$ (7.1)	$ (26.9)