REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Regulatory assets
Other current assets	$ 0.0	$ 0.1
Regulatory assets	3,134.2	2,902.2
Total regulatory assets	3,134.2	2,902.3
Plant retirements
Regulatory assets
Total regulatory assets	944.5	754.1
Finance and capital leases
Regulatory assets
Total regulatory assets	915.8	869.3
Pension and OPEB costs
Regulatory assets
Total regulatory assets	468.0	490.6
Income tax related items
Regulatory assets
Total regulatory assets	398.8	317.9
System support resource
Regulatory assets
Total regulatory assets	320.1	316.7
We Power generation
Regulatory assets
Total regulatory assets	29.9	43.0
Environmental remediation costs
Regulatory assets
Total regulatory assets	23.4	24.2
Electric transmission costs
Regulatory assets
Total regulatory assets	16.3	57.8
Other, net
Regulatory assets
Total regulatory assets	$ 17.4	$ 28.7