COMMITMENTS AND CONTINGENCIES - ENVIRONMENTAL MATTERS (Details) T in Millions, $ in Millions	3 Months Ended	12 Months Ended	15 Months Ended
	Mar. 31, 2021 USD ($) States performance_obligations compliance_option MW	Dec. 31, 2020 USD ($) T	Mar. 31, 2019 MW
Manufactured gas plant remediation
Regulatory assets	$ 3,793.8	$ 3,544.1
Environmental remediation costs
Manufactured gas plant remediation
Regulatory assets	$ 623.1	$ 638.2
National Ambient Air Quality Standards | Electric
Air quality
Number of changes to the 2015 ozone standards | performance_obligations	0
Number of revisions necessary to the 2012 standard for particulate matter | performance_obligations	0
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 percentage of carbon dioxide emissions reduction below 2005 levels by 2025	60.00%
Company goal for percentage of carbon dioxide emissions reduction below 2005 levels by 2030	80.00%
Capacity of coal-fired generation retired, in megawatts | MW			1,800
Capacity of fossil-fueled generation to be retired by 2025, in megawatts | MW	1,800
Carbon dioxide emissions | T		20.1
Climate Change | Natural gas
Air quality
Initial 2030 percentage goal for a reduction of methane emissions from a 2011 baseline	30.00%
Carbon dioxide emissions | T		27.0
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
Number of new ELG rule requirements that affect our electric utilities | performance_obligations	2
Expected capital investment to achieve required discharge limits	$ 110.0
Manufactured Gas Plant Remediation | Natural gas
Manufactured gas plant remediation
Reserves for future environmental remediation	532.9	$ 532.9
Manufactured Gas Plant Remediation | Natural gas | Environmental remediation costs
Manufactured gas plant remediation
Regulatory assets	$ 623.1	$ 638.2