OPERATING REVENUES - DISAGGREGATION OF NATURAL GAS UTILITY OPERATING REVENUES BY CUSTOMER CLASS (Details) - Revenues from contracts with customers - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 1,654.3	$ 1,543.0	$ 4,323.3	$ 3,629.9
Natural gas
Disaggregation of Operating Revenues
Revenues from contracts with customers	546.4	525.2	2,094.1	1,557.9
Total Utility Operations | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,633.3	1,528.5	4,279.4	3,599.8
Total Utility Operations | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	545.7	524.7	2,092.1	1,557.0
Total Utility Operations | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	584.6	423.3	1,677.0	1,409.0
Total Utility Operations | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	425.6	323.2	1,195.0	1,014.5
Total Utility Operations | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	159.0	100.1	482.0	394.5
Total Utility Operations | Natural gas | Transferred over time | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	73.2	70.3	182.8	177.6
Total Utility Operations | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	(112.1)	31.1	232.3	(29.6)
Wisconsin | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,295.9	1,207.4	3,018.2	2,700.9
Wisconsin | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	212.7	207.8	840.0	666.7
Wisconsin | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	278.8	172.8	802.8	637.2
Wisconsin | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	188.6	125.8	536.2	438.9
Wisconsin | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	90.2	47.0	266.6	198.3
Wisconsin | Natural gas | Transferred over time | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	17.8	17.0	42.2	41.1
Wisconsin | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	(83.9)	18.0	(5.0)	(11.6)
Illinois | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	266.1	254.5	959.6	688.1
Illinois | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	266.1	254.5	959.6	688.1
Illinois | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	250.6	193.9	687.2	568.2
Illinois | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	199.1	157.4	533.0	440.3
Illinois | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	51.5	36.5	154.2	127.9
Illinois | Natural gas | Transferred over time | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	48.8	46.3	123.0	119.0
Illinois | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	(33.3)	14.3	149.4	0.9
Other States | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	71.3	66.6	301.6	210.8
Other States | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	66.9	62.4	292.5	202.2
Other States | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	55.2	56.6	187.0	203.6
Other States | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	37.9	40.0	125.8	135.3
Other States | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	17.3	16.6	61.2	68.3
Other States | Natural gas | Transferred over time | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	6.6	7.0	17.6	17.5
Other States | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 5.1	$ (1.2)	$ 87.9	$ (18.9)