Leases - Finance and Operating Lease Right of Use Assets and Obligations (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Leases
Operating lease right of use assets	$ 32.0	$ 15.7
Finance lease right of use assets	133.8	175.3
Current operating lease liabilities	4.7	4.0
Long-term operating lease liabilities	38.8	25.4
Current finance lease liabilities	0.0
Long-term finance lease liabilities	145.9	110.5
Accumulated amortization	$ 6.1	$ 146.3
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]	Other	Other
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]	Property, plant, and equipment, net of accumulated depreciation and amortization of $11,073.1 and $10,383.8, respectively	Property, plant, and equipment, net of accumulated depreciation and amortization of $11,073.1 and $10,383.8, respectively
Operating Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration]	Other	Other
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible Enumeration]	Other	Other
Finance Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration]	Current portion of long-term debt (December 31, 2023 and December 31, 2022 include $9.0 and $8.9, respectively, related to WEPCo Environmental Trust)	Current portion of long-term debt (December 31, 2023 and December 31, 2022 include $9.0 and $8.9, respectively, related to WEPCo Environmental Trust)
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible Enumeration]	Long-term debt (December 31, 2023 and December 31, 2022 include $85.3 and $94.1, respectively, related to WEPCo Environmental Trust)	Long-term debt (December 31, 2023 and December 31, 2022 include $85.3 and $94.1, respectively, related to WEPCo Environmental Trust)
Power purchase commitment
Leases
Finance lease right of use assets	$ 0.0	$ 71.8
Current finance lease liabilities	0.0	72.7
Land lease - utility solar generation
Leases
Finance lease right of use assets	132.7	102.4
Current finance lease liabilities	0.0
Long-term finance lease liabilities	144.8	109.3
Other
Leases
Finance lease right of use assets	1.1	1.1
Current finance lease liabilities	0.0
Long-term finance lease liabilities	$ 1.1	$ 1.2