REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Regulatory assets
Regulatory assets	$ 432.2	$ 449.7
Plant retirement related items
Regulatory assets
Regulatory assets	134.2	143.6
Environmental remediation costs
Regulatory assets
Regulatory assets	109.5	114.0
Income tax related items
Regulatory assets
Regulatory assets	57.3	55.1
Asset retirement obligations
Regulatory assets
Regulatory assets	35.6	27.5
Pension and OPEB costs
Regulatory assets
Regulatory assets	32.6	45.6
Bluewater Natural Gas Holding, LLC
Regulatory assets
Regulatory assets	11.5	13.4
Reactive power
Regulatory assets
Regulatory assets	10.0	13.4
Electric transmission costs
Regulatory assets
Regulatory assets	8.7	0.4
Finance and operating leases
Regulatory assets
Regulatory assets	8.6	7.5
Uncollectible expense
Regulatory assets
Regulatory assets	5.4	9.7
Other, net
Regulatory assets
Regulatory assets	$ 18.8	$ 19.5