COMMITMENTS AND CONTINGENCIES - ENVIRONMENTAL MATTERS (Details) T in Millions, $ in Millions	1 Months Ended				3 Months Ended	12 Months Ended		15 Months Ended
	Mar. 31, 2022 USD ($) micrograms MW	Jun. 30, 2021 area	Dec. 31, 2020 USD ($) micrograms performance_obligations	Jun. 30, 2020 generating_units	Mar. 31, 2022 USD ($) States MW	Dec. 31, 2021 USD ($) T	Dec. 31, 2020 USD ($) performance_obligations	Mar. 31, 2019 MW	Oct. 31, 2021 performance_obligations
Manufactured gas plant remediation
Regulatory assets | $	$ 3,255.6				$ 3,255.6	$ 3,367.1
Environmental remediation costs
Manufactured gas plant remediation
Regulatory assets | $	$ 618.8				$ 618.8	$ 630.9
Cross State Air Pollution Rule - Good Neighbor Plan | Electric
Air quality
Number of states the EPA is proposing to update and expand cross state air pollution rules to regulate NOx emissions in | States					25
Cross State Air Pollution Rule - Good Neighbor Plan | Electric | Maximum
Air quality
RICE unit megawatts | MW	25				25
Cross State Air Pollution Rule - Good Neighbor Plan | Electric | Minimum
Air quality
RICE unit megawatts | MW	15				15
National Ambient Air Quality Standards | Electric
Air quality
Number of changes to the 2015 ozone standards | performance_obligations									0
Number of counties to have boundaries revised | area		13
Number of nonattainment areas as designated by the EPA | area		6
Number of revisions necessary to meet the 2012 standard for particulate matter | performance_obligations			0
Current number of micrograms per cubic meter that particulate matter needs to be below			12
Lowest limit that will cause non-attainment	10
National Ambient Air Quality Standards | Electric | Maximum
Air quality
Majority of CASAC members support this range in the peer review they completed	10
Minority of CASAC members support this range in the peer review they completed	11
National Ambient Air Quality Standards | Electric | Minimum
Air quality
Majority of CASAC members support this range in the peer review they completed	8
Minority of CASAC members support this range in the peer review they completed	10
Climate Change | Electric
Air quality
Capacity of coal-fired generation retired, in megawatts | MW								1,800
Capacity of fossil-fueled generation to be retired by the end of 2025, in megawatts | MW					1,600
Company goal for percentage of carbon emissions reduction below 2005 levels by the end of 2025					60.00%
Company goal for percentage of carbon emissions reduction below 2005 levels by the end of 2030					80.00%
Carbon dioxide emissions | T						22.0
Climate Change | Natural gas
Air quality
Carbon dioxide emissions | T						26.0
Clean Water Act Cooling Water Intake Structure Rule | Electric
Water quality
Number of generating units to be retired | generating_units				4
Steam Electric Effluent Limitation Guidelines | Electric
Water quality
Number of new ELG rule requirements that affect our electric utilities | performance_obligations							2
Additional capital investment to achieve required discharge limits | $			$ 100.0				$ 100.0
Manufactured Gas Plant Remediation | Natural gas
Manufactured gas plant remediation
Reserves for future environmental remediation | $	$ 517.2				$ 517.2	$ 532.6
Manufactured Gas Plant Remediation | Natural gas | Environmental remediation costs
Manufactured gas plant remediation
Regulatory assets | $	$ 618.8				$ 618.8	$ 630.9