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, 2019	Mar. 31, 2018
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 2,376.5	$ 2,290.0
Natural gas
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,296.4	1,210.7
Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,294.7	1,198.3
Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,251.0	1,174.5
Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	863.1	812.5
Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	387.9	362.0
Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	120.2	108.6
Total Utility Operations | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	2,360.7	2,269.9
Total Utility Operations | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,294.7	1,198.3
Total Utility Operations | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	(76.5)	(84.8)
Wisconsin | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,626.7	1,585.7
Wisconsin | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	564.9	518.0
Wisconsin | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	583.6	544.6
Wisconsin | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	383.9	356.7
Wisconsin | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	199.7	187.9
Wisconsin | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	21.9	21.0
Wisconsin | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	(40.6)	(47.6)
Illinois | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	544.7	507.6
Illinois | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	544.6	507.6
Illinois | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	470.2	442.0
Illinois | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	354.0	332.6
Illinois | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	116.2	109.4
Illinois | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	87.2	77.7
Illinois | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	(12.8)	(12.1)
Other States | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	189.3	176.6
Other States | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	185.2	172.7
Other States | Natural gas | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	197.2	187.9
Other States | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	125.2	123.2
Other States | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	72.0	64.7
Other States | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	11.1	9.9
Other States | Natural gas | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ (23.1)	$ (25.1)