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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, 2020
Dec. 31, 2019
Dec. 31, 2018
Disaggregation of Operating Revenues      
Revenues from contracts with customers $ 7,169.3 $ 7,449.3 $ 7,680.3
Natural gas      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 2,826.9 3,071.4 3,194.9
Natural gas | Transferred over time      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 2,824.5 3,068.1 3,185.9
Natural gas | Transferred over time | Total retail revenues      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 2,450.5 2,784.2 2,818.9
Natural gas | Transferred over time | Residential      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 1,775.6 1,953.9 1,975.3
Natural gas | Transferred over time | Commercial and industrial      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 674.9 830.3 843.6
Natural gas | Transferred over time | Transport      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 326.2 349.5 346.7
Natural gas | Transferred over time | Other utility revenues      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 47.8 (65.6) 20.3
Wisconsin | Transferred over time      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 5,461.7 5,631.8 5,783.0
Wisconsin | Natural gas | Transferred over time      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 1,195.6 1,324.1 1,350.6
Wisconsin | Natural gas | Transferred over time | Total retail revenues      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 1,090.7 1,257.8 1,271.2
Wisconsin | Natural gas | Transferred over time | Residential      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 752.6 837.9 834.5
Wisconsin | Natural gas | Transferred over time | Commercial and industrial      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 338.1 419.9 436.7
Wisconsin | Natural gas | Transferred over time | Transport      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 79.1 72.6 70.8
Wisconsin | Natural gas | Transferred over time | Other utility revenues      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 25.8 (6.3) 8.6
Illinois | Transferred over time      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 1,267.9 1,332.5 1,407.1
Illinois | Natural gas | Transferred over time      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 1,267.9 1,332.4 1,406.9
Illinois | Natural gas | Transferred over time | Total retail revenues      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 1,023.2 1,119.5 1,144.4
Illinois | Natural gas | Transferred over time | Residential      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 802.2 857.8 877.5
Illinois | Natural gas | Transferred over time | Commercial and industrial      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 221.0 261.7 266.9
Illinois | Natural gas | Transferred over time | Transport      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 215.6 245.3 244.1
Illinois | Natural gas | Transferred over time | Other utility revenues      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 29.1 (32.4) 18.4
Other States | Transferred over time      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 378.1 428.2 444.5
Other States | Natural gas | Transferred over time      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 361.0 411.6 428.4
Other States | Natural gas | Transferred over time | Total retail revenues      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 336.6 406.9 403.3
Other States | Natural gas | Transferred over time | Residential      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 220.8 258.2 263.3
Other States | Natural gas | Transferred over time | Commercial and industrial      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 115.8 148.7 140.0
Other States | Natural gas | Transferred over time | Transport      
Disaggregation of Operating Revenues      
Revenues from contracts with customers 31.5 31.6 31.8
Other States | Natural gas | Transferred over time | Other utility revenues      
Disaggregation of Operating Revenues      
Revenues from contracts with customers $ (7.1) $ (26.9) $ (6.7)