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Operating Revenues - Disaggregation Of Operating Revenues by Segment (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Operating Revenues
Operating revenues	$ 2,076.8	$ 1,643.7	$ 1,672.5	$ 2,286.5	$ 2,055.0	$ 1,657.5	$ 1,631.5	$ 2,304.5	$ 7,679.5	$ 7,648.5	$ 7,472.3
Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									7,680.3
Other operating revenues
Disaggregation of Operating Revenues
Operating revenues									(0.8)
Total utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									7,627.3
Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									4,432.4
Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									3,194.9
Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									3,185.9
Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									53.0
Total Utility Operations
Disaggregation of Operating Revenues
Operating revenues									7,632.9
Total Utility Operations | Other operating revenues
Disaggregation of Operating Revenues
Operating revenues									(1.7)
Total Utility Operations | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									7,634.6
Total Utility Operations | Total utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									7,618.3
Total Utility Operations | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									4,432.4
Total Utility Operations | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									3,185.9
Total Utility Operations | Other non-utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									16.3
Wisconsin
Disaggregation of Operating Revenues
Operating revenues									5,794.7
Wisconsin | Other operating revenues
Disaggregation of Operating Revenues
Operating revenues									11.7
Wisconsin | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									5,783.0
Wisconsin | Total utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									5,783.0
Wisconsin | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									4,432.4
Wisconsin | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									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
Illinois
Disaggregation of Operating Revenues
Operating revenues									1,400.0
Illinois | Other operating revenues
Disaggregation of Operating Revenues
Operating revenues									(7.1)
Illinois | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									1,407.1
Illinois | Total utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									1,406.9
Illinois | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									0.0
Illinois | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									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									0.2
Other States
Disaggregation of Operating Revenues
Operating revenues									438.2
Other States | Other operating revenues
Disaggregation of Operating Revenues
Operating revenues									(6.3)
Other States | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									444.5
Other States | Total utility revenues | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									428.4
Other States | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									0.0
Other States | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									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									16.1
Electric transmission
Disaggregation of Operating Revenues
Operating revenues									0.0
Electric transmission | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									0.0
Electric transmission | Other operating revenues
Disaggregation of Operating Revenues
Operating revenues									0.0
Electric transmission | Total utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									0.0
Electric transmission | Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									0.0
Electric transmission | Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									0.0
Electric transmission | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									0.0
Non-Utility Energy Infrastructure
Disaggregation of Operating Revenues
Operating revenues									468.4
Non-Utility Energy Infrastructure | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									80.0
Non-Utility Energy Infrastructure | Other operating revenues
Disaggregation of Operating Revenues
Operating revenues									388.4
Non-Utility Energy Infrastructure | Total utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									45.4
Non-Utility Energy Infrastructure | Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									0.0
Non-Utility Energy Infrastructure | Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									45.4
Non-Utility Energy Infrastructure | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									34.6
Corporate and Other
Disaggregation of Operating Revenues
Operating revenues									8.7
Corporate and Other | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									7.9
Corporate and Other | Other operating revenues
Disaggregation of Operating Revenues
Operating revenues									0.8
Corporate and Other | Total utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									0.0
Corporate and Other | Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									0.0
Corporate and Other | Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									0.0
Corporate and Other | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									7.9
Reconciling Eliminations
Disaggregation of Operating Revenues
Operating revenues									(430.5)
Reconciling Eliminations | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									(42.2)
Reconciling Eliminations | Other operating revenues
Disaggregation of Operating Revenues
Operating revenues									(388.3)
Reconciling Eliminations | Total utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									(36.4)
Reconciling Eliminations | Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									0.0
Reconciling Eliminations | Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									(36.4)
Reconciling Eliminations | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									$ (5.8)

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