Operating Revenues - Disaggregation Of Operating Revenues by Segment (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Operating Revenues
Operating revenues	$ 1,933.4	$ 1,651.0	$ 1,548.7	$ 2,108.6	$ 1,947.5	$ 1,608.0	$ 1,590.2	$ 2,377.4	$ 7,241.7	$ 7,523.1	$ 7,679.5
Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									7,169.3	7,449.3	7,680.3
Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues									72.4	73.8	(0.8)
Total regulated revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									7,093.0	7,379.1	7,627.3
Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									4,266.1	4,307.7	4,432.4
Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									2,826.9	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									2,824.5	3,068.1	3,185.9
Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									76.3	70.2	53.0
Reconciling Eliminations
Disaggregation of Operating Revenues
Operating revenues									(448.5)	(407.4)	(430.5)
Reconciling Eliminations | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									(51.1)	(49.8)	(42.2)
Reconciling Eliminations | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues									(397.4)	(357.6)	(388.3)
Reconciling Eliminations | Total regulated revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									(42.0)	(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	0.0
Reconciling Eliminations | Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									(42.0)	(44.1)	(36.4)
Reconciling Eliminations | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									(9.1)	(5.7)	(5.8)
Total Utility Operations
Disaggregation of Operating Revenues
Operating revenues									7,179.5	7,430.2	7,632.9
Total Utility Operations | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues									71.8	37.7	(1.7)
Total Utility Operations | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									7,107.7	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									7,090.6	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									4,266.1	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									2,824.5	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									17.1	16.7	16.3
Wisconsin
Disaggregation of Operating Revenues
Operating revenues									5,473.5	5,647.1	5,794.7
Wisconsin | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues									11.8	15.3	11.7
Wisconsin | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									5,461.7	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									5,461.7	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									4,266.1	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									1,195.6	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	0.0
Illinois
Disaggregation of Operating Revenues
Operating revenues									1,321.9	1,357.1	1,400.0
Illinois | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues									54.0	24.6	(7.1)
Illinois | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									1,267.9	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									1,267.9	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	0.0
Illinois | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									1,267.9	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									0.0	0.1	0.2
Other States
Disaggregation of Operating Revenues
Operating revenues									384.1	426.0	438.2
Other States | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues									6.0	(2.2)	(6.3)
Other States | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									378.1	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									361.0	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	0.0
Other States | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									361.0	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									17.1	16.6	16.1
Non-Utility Energy Infrastructure
Disaggregation of Operating Revenues
Operating revenues									508.5	495.9	468.4
Non-Utility Energy Infrastructure | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									111.0	102.6	80.0
Non-Utility Energy Infrastructure | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues									397.5	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									44.4	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	0.0
Non-Utility Energy Infrastructure | Natural gas | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									44.4	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									66.6	55.2	34.6
Corporate and Other
Disaggregation of Operating Revenues
Operating revenues									2.2	4.4	8.7
Corporate and Other | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									1.7	4.0	7.9
Corporate and Other | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues									0.5	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	0.0
Corporate and Other | Electric | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									0.0	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	0.0
Corporate and Other | Other non-utility revenues | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers									$ 1.7	$ 4.0	$ 7.9