REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Regulatory assets
Amounts recoverable from customers	$ 306.7	$ 20.0
Regulatory assets	3,487.1	3,524.1
Total regulatory assets	3,793.8	3,544.1
Pension and OPEB costs
Regulatory assets
Total regulatory assets	1,073.9	1,101.6
Plant retirements
Regulatory assets
Total regulatory assets	733.1	740.8
Environmental remediation costs
Regulatory assets
Total regulatory assets	623.1	638.2
Income tax related items
Regulatory assets
Total regulatory assets	459.3	454.6
Energy costs recoverable through rate adjustments
Regulatory assets
Total regulatory assets	301.6	1.1
Asset retirement obligations
Regulatory assets
Total regulatory assets	186.0	181.3
SSR
Regulatory assets
Total regulatory assets	134.6	135.6
Securitization
Regulatory assets
Total regulatory assets	106.9	105.2
Uncollectible expense
Regulatory assets
Total regulatory assets	73.0	82.0
Derivatives
Regulatory assets
Total regulatory assets	9.6	26.5
Other, net
Regulatory assets
Total regulatory assets	$ 92.7	$ 77.2