OPERATING REVENUES - DISAGGREGATION OF NATURAL GAS UTILITY OPERATING REVENUES BY CUSTOMER CLASS (Details) - Revenues from contracts with customers - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 2,899.6	$ 2,669.0
Natural gas
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,665.4	1,547.7
Total Utility Operations | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	2,857.0	2,646.1
Total Utility Operations | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,664.9	1,546.4
Total Utility Operations | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,646.9	1,092.4
Total Utility Operations | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,129.3	769.4
Total Utility Operations | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	517.6	323.0
Total Utility Operations | Natural gas | Transferred over time | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	120.3	109.6
Total Utility Operations | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	(102.3)	344.4
Wisconsin | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,934.3	1,722.3
Wisconsin | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	746.8	627.3
Wisconsin | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	775.0	524.0
Wisconsin | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	502.5	347.6
Wisconsin | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	272.5	176.4
Wisconsin | Natural gas | Transferred over time | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	25.5	24.4
Wisconsin | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	(53.7)	78.9
Illinois | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	679.9	693.5
Illinois | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	679.9	693.5
Illinois | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	623.8	436.6
Illinois | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	465.5	333.9
Illinois | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	158.3	102.7
Illinois | Natural gas | Transferred over time | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	80.9	74.2
Illinois | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	(24.8)	182.7
Other States | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	242.8	230.3
Other States | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	238.2	225.6
Other States | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	248.1	131.8
Other States | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	161.3	87.9
Other States | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	86.8	43.9
Other States | Natural gas | Transferred over time | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	13.9	11.0
Other States | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ (23.8)	$ 82.8