COMMITMENTS AND CONTINGENCIES - ENVIRONMENTAL MATTERS (Details) T in Millions, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) generating_units change degreecelsius MW	Dec. 31, 2018 USD ($) T
Manufactured gas plant remediation
Regulatory assets	$ 4,037.1	$ 3,855.8
Environmental remediation costs
Manufactured gas plant remediation
Regulatory assets	$ 697.6	$ 687.1
Mercury and Air Toxics Standards
Air quality
Revisions to Mercury and Air Toxics Standards | change	0
Climate Change | Electric
Air quality
Company goal for percentage of carbon dioxide emissions reduction	40.00%
Long-term company goal for percentage of carbon dioxide emissions reduction by 2050	80.00%
Capacity of coal generation retired since the beginning of 2018 | MW	1,800
Carbon dioxide emissions | T		26.4
Climate Change | Electric | Maximum
Air quality
Global temperature increases limit | degreecelsius	2
Climate Change | Natural gas
Air quality
Carbon dioxide emissions | T		29.4
Steam Electric Effluent Limitation Guidelines | Electric
Water quality
Number of generating units of OCPP and ERGS | generating_units	6
Expected cost to achieve required emissions reductions	$ 70.0
Manufactured Gas Plant Remediation | Natural gas
Manufactured gas plant remediation
Reserves for future remediation	631.8	$ 616.4
Manufactured Gas Plant Remediation | Natural gas | Environmental remediation costs
Manufactured gas plant remediation
Regulatory assets	$ 697.6	$ 687.1