REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Regulatory assets
Total regulatory assets	$ 2,790.6	$ 2,803.3
Finance leases
Regulatory assets
Total regulatory assets	1,010.2	985.5
Plant retirements
Regulatory assets
Total regulatory assets	661.7	669.8
Pension and OPEB costs
Regulatory assets
Total regulatory assets	457.4	477.0
Income tax related items
Regulatory assets
Total regulatory assets	390.4	392.6
SSR
Regulatory assets
Total regulatory assets	132.8	135.6
Securitization
Regulatory assets
Total regulatory assets	106.0	105.2
Asset retirement obligations
Regulatory assets
Total regulatory assets	27.9	28.6
Other, net
Regulatory assets
Total regulatory assets	$ 4.2	$ 9.0