COMMITMENTS AND CONTINGENCIES - ENVIRONMENTAL MATTERS (Details) $ in Millions	1 Months Ended			9 Months Ended	15 Months Ended
	Jun. 30, 2021 area	Mar. 31, 2021 performance_obligations States	Dec. 31, 2020 USD ($) performance_obligations micrograms	Sep. 30, 2021 USD ($) MW	Mar. 31, 2019 MW	Feb. 28, 2021 compliance_option
Manufactured gas plant remediation
Regulatory assets			$ 449.4	$ 414.8
Environmental remediation costs
Manufactured gas plant remediation
Regulatory assets			$ 116.1	$ 113.0
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 | 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 | micrograms			12
Climate Change | Electric
Air quality
Number of rules that regulate GHG emissions from electric generating units | compliance_option						0
Company goal percentage met by the end of 2020 for carbon dioxide emissions reduction below 2005 levels			50.00%
Company goal for percent of carbon emission reduction below 2005 levels by 2025				60.00%
Company goal for percentage of carbon emission reduction below 2005 levels by 2030				80.00%
Capacity of coal-fired generation retired, in megawatts | MW					300
Capacity of fossil-fueled generation to be retired by 2025, in megawatts | MW				1,600
Climate Change | Natural gas
Air quality
Initial 2030 percentage goal for a reduction of methane emissions from a 2011 baseline				30.00%
Cross State Air Pollution Rule Update Rule Revision | Electric
Air quality
Number of states in group 2 for CSAPR update rule revision | States		9
Number of states affected by the CSAPR update rule revision | States		21
Amount of further NOx reductions needed within the nine effected states | performance_obligations		0
Steam Electric Effluent Limitation Guidelines | Electric
Water quality
Expected capital investment to achieve required discharge limits				$ 10.0
Manufactured Gas Plant Remediation | Natural gas
Manufactured gas plant remediation
Reserves for future environmental remediation			$ 88.3	87.3
Manufactured Gas Plant Remediation | Natural gas | Environmental remediation costs
Manufactured gas plant remediation
Regulatory assets			$ 116.1	$ 113.0