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Operating Revenues - Disaggregation Of Operating Revenues by Segment (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disaggregation of Operating Revenues                      
Operating revenues $ 1,947.5 $ 1,608.0 $ 1,590.2 $ 2,377.4 $ 2,076.8 $ 1,643.7 $ 1,672.5 $ 2,286.5 $ 7,523.1 $ 7,679.5 $ 7,648.5
Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 7,449.3 7,680.3  
Other operating revenues                      
Disaggregation of Operating Revenues                      
Operating revenues                 73.8 (0.8)  
Total regulated revenues | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 7,379.1 7,627.3  
Electric | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 4,307.7 4,432.4  
Natural gas | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 3,071.4 3,194.9  
Natural gas | Transferred over time | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 3,068.1 3,185.9  
Other non-utility revenues | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 70.2 53.0  
Total Utility Operations                      
Disaggregation of Operating Revenues                      
Operating revenues                 7,430.2 7,632.9  
Total Utility Operations | Other operating revenues                      
Disaggregation of Operating Revenues                      
Operating revenues                 37.7 (1.7)  
Total Utility Operations | Transferred over time | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 7,392.5 7,634.6  
Total Utility Operations | Total regulated revenues | Transferred over time | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 7,375.8 7,618.3  
Total Utility Operations | Electric | Transferred over time | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 4,307.7 4,432.4  
Total Utility Operations | Natural gas | Transferred over time | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 3,068.1 3,185.9  
Total Utility Operations | Other non-utility revenues | Transferred over time | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 16.7 16.3  
Wisconsin                      
Disaggregation of Operating Revenues                      
Operating revenues                 5,647.1 5,794.7  
Wisconsin | Other operating revenues                      
Disaggregation of Operating Revenues                      
Operating revenues                 15.3 11.7  
Wisconsin | Transferred over time | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 5,631.8 5,783.0  
Wisconsin | Total regulated revenues | Transferred over time | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 5,631.8 5,783.0  
Wisconsin | Electric | Transferred over time | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 4,307.7 4,432.4  
Wisconsin | Natural gas | Transferred over time | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 1,324.1 1,350.6  
Wisconsin | Other non-utility revenues | Transferred over time | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 0.0 0.0  
Illinois                      
Disaggregation of Operating Revenues                      
Operating revenues                 1,357.1 1,400.0  
Illinois | Other operating revenues                      
Disaggregation of Operating Revenues                      
Operating revenues                 24.6 (7.1)  
Illinois | Transferred over time | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 1,332.5 1,407.1  
Illinois | Total regulated revenues | Transferred over time | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 1,332.4 1,406.9  
Illinois | Electric | Transferred over time | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 0.0 0.0  
Illinois | Natural gas | Transferred over time | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 1,332.4 1,406.9  
Illinois | Other non-utility revenues | Transferred over time | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 0.1 0.2  
Other States                      
Disaggregation of Operating Revenues                      
Operating revenues                 426.0 438.2  
Other States | Other operating revenues                      
Disaggregation of Operating Revenues                      
Operating revenues                 (2.2) (6.3)  
Other States | Transferred over time | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 428.2 444.5  
Other States | Total regulated revenues | Transferred over time | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 411.6 428.4  
Other States | Electric | Transferred over time | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 0.0 0.0  
Other States | Natural gas | Transferred over time | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 411.6 428.4  
Other States | Other non-utility revenues | Transferred over time | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 16.6 16.1  
Non-Utility Energy Infrastructure                      
Disaggregation of Operating Revenues                      
Operating revenues                 495.9 468.4  
Non-Utility Energy Infrastructure | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 102.6 80.0  
Non-Utility Energy Infrastructure | Other operating revenues                      
Disaggregation of Operating Revenues                      
Operating revenues                 393.3 388.4  
Non-Utility Energy Infrastructure | Total regulated revenues | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 47.4 45.4  
Non-Utility Energy Infrastructure | Electric | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 0.0 0.0  
Non-Utility Energy Infrastructure | Natural gas | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 47.4 45.4  
Non-Utility Energy Infrastructure | Other non-utility revenues | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 55.2 34.6  
Corporate and Other                      
Disaggregation of Operating Revenues                      
Operating revenues                 4.4 8.7  
Corporate and Other | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 4.0 7.9  
Corporate and Other | Other operating revenues                      
Disaggregation of Operating Revenues                      
Operating revenues                 0.4 0.8  
Corporate and Other | Total regulated revenues | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 0.0 0.0  
Corporate and Other | Electric | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 0.0 0.0  
Corporate and Other | Natural gas | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 0.0 0.0  
Corporate and Other | Other non-utility revenues | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 4.0 7.9  
Reconciling Eliminations                      
Disaggregation of Operating Revenues                      
Operating revenues                 (407.4) (430.5)  
Reconciling Eliminations | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 (49.8) (42.2)  
Reconciling Eliminations | Other operating revenues                      
Disaggregation of Operating Revenues                      
Operating revenues                 (357.6) (388.3)  
Reconciling Eliminations | Total regulated revenues | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 (44.1) (36.4)  
Reconciling Eliminations | Electric | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 0.0 0.0  
Reconciling Eliminations | Natural gas | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 (44.1) (36.4)  
Reconciling Eliminations | Other non-utility revenues | Revenues from contracts with customers                      
Disaggregation of Operating Revenues                      
Revenues from contracts with customers                 $ (5.7) $ (5.8)