REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Regulatory assets
Total regulatory assets	$ 2,809.1	$ 2,803.3
Finance leases
Regulatory assets
Total regulatory assets	1,021.9	985.5
Plant retirements
Regulatory assets
Total regulatory assets	661.1	669.8
Pension and OPEB costs
Regulatory assets
Total regulatory assets	446.8	477.0
Income tax related items
Regulatory assets
Total regulatory assets	388.7	392.6
SSR
Regulatory assets
Total regulatory assets	131.6	135.6
Securitization
Regulatory assets
Total regulatory assets	102.9	105.2
AROs
Regulatory assets
Total regulatory assets	43.7	28.6
Other, net
Regulatory assets
Total regulatory assets	$ 12.4	$ 9.0