REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Regulatory assets
Total regulatory assets	$ 2,961.2	$ 2,941.0
We Power finance leases
Regulatory assets
Total regulatory assets	1,166.3	1,158.6
Plant retirement related items
Regulatory assets
Total regulatory assets	601.1	634.3
Income tax related items
Regulatory assets
Total regulatory assets	465.2	413.3
Pension and OPEB costs
Regulatory assets
Total regulatory assets	262.6	289.0
System support resource
Regulatory assets
Total regulatory assets	85.0	92.6
Electric transmission costs
Regulatory assets
Total regulatory assets	71.0	15.3
Asset retirement obligations
Regulatory assets
Total regulatory assets	67.6	59.7
Securitization
Regulatory assets
Total regulatory assets	60.9	67.5
Uncollectible expense
Regulatory assets
Total regulatory assets	49.3	114.7
Other finance and operating leases
Regulatory assets
Total regulatory assets	34.9	25.2
Derivatives
Regulatory assets
Total regulatory assets	19.5	9.0
Revenue requirements of generation facilities
Regulatory assets
Total regulatory assets	16.6	3.6
Bluewater Natural Gas Holding, LLC
Regulatory assets
Total regulatory assets	8.3	13.9
Other, net
Regulatory assets
Total regulatory assets	$ 52.9	$ 44.3