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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 1,543.0	$ 1,563.9	$ 3,629.9	$ 3,940.4
Natural gas
Disaggregation of Operating Revenues
Revenues from contracts with customers	525.2	524.8	1,557.9	1,821.2
Total Utility Operations | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,528.5	1,550.2	3,599.8	3,910.9
Total Utility Operations | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	524.7	524.1	1,557.0	1,818.8
Total Utility Operations | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	423.3	441.0	1,409.0	1,692.0
Total Utility Operations | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	323.2	319.6	1,014.5	1,182.7
Total Utility Operations | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	100.1	121.4	394.5	509.3
Total Utility Operations | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	70.3	69.3	177.6	189.5
Total Utility Operations | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	31.1	13.8	(29.6)	(62.7)
Wisconsin | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,207.4	1,249.2	2,700.9	2,875.9
Wisconsin | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	207.8	227.3	666.7	792.2
Wisconsin | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	172.8	182.0	637.2	765.6
Wisconsin | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	125.8	126.2	438.9	510.1
Wisconsin | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	47.0	55.8	198.3	255.5
Wisconsin | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	17.0	16.2	41.1	38.1
Wisconsin | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	18.0	29.1	(11.6)	(11.5)
Illinois | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	254.5	232.8	688.1	777.5
Illinois | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	254.5	232.8	688.1	777.4
Illinois | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	193.9	199.2	568.2	669.4
Illinois | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	157.4	154.1	440.3	508.1
Illinois | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	36.5	45.1	127.9	161.3
Illinois | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	46.3	46.8	119.0	134.0
Illinois | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	14.3	(13.2)	0.9	(26.0)
Other States | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	66.6	68.2	210.8	257.5
Other States | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	62.4	64.0	202.2	249.2
Other States | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	56.6	59.8	203.6	257.0
Other States | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	40.0	39.3	135.3	164.5
Other States | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	16.6	20.5	68.3	92.5
Other States | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	7.0	6.3	17.5	17.4
Other States | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ (1.2)	$ (2.1)	$ (18.9)	$ (25.2)