REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Regulatory assets
Current assets	$ 27.3	$ 50.7
Regulatory assets	4,009.8	3,805.1
Total regulatory assets	4,037.1	3,855.8
Pension and OPEB costs
Regulatory assets
Total regulatory assets	1,173.8	1,193.5
Plant retirements
Regulatory assets
Total regulatory assets	1,027.9	832.3
Environmental remediation costs
Regulatory assets
Total regulatory assets	697.6	687.1
Income tax related items
Regulatory assets
Total regulatory assets	384.8	369.1
System support resource
Regulatory assets
Total regulatory assets	317.9	316.7
Asset retirement obligations
Regulatory assets
Total regulatory assets	223.4	185.4
Uncollectible expense
Regulatory assets
Total regulatory assets	47.6	38.7
Electric transmission costs
Regulatory assets
Total regulatory assets	41.7	58.1
We Power generation
Regulatory assets
Total regulatory assets	36.1	43.0
Energy efficiency programs
Regulatory assets
Total regulatory assets	11.3	14.0
Other, net
Regulatory assets
Total regulatory assets	$ 75.0	$ 117.9