Commitments and Contingencies, Leases (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Lessee, Lease, Description [Line Items]
Operating Lease, Weighted Average Discount Rate, Percent		4.40%
Short-term Lease, Cost		$ 1.5	$ 1.1	$ 1.2
Operating Lease, Assets and Liabilities, Lessee [Abstract]
Operating Lease, Right-of-Use Asset, Gross		548.3
Operating Lease, Right-of-Use Asset, Accumulated Depreciation		(25.9)
Operating lease right-of-use assets		522.4	0.0
Lease, Cost [Abstract]
Operating Lease, Cost	[1]	53.0	59.0	57.8
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
2020		49.6
2021		49.5
2022		49.6
2023		49.6
2024		49.7
Thereafter		455.8
Total minimum obligation		703.8
Interest component of obligation		(181.6)
Present value of minimum obligation		522.2
Less current portion		(26.9)	0.0
Operating Lease, Liability, Noncurrent		$ 495.3	0.0
Weighted-average remaining lease term in years		14 years 1 month 6 days
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019			51.9
2020			51.4
2021			51.3
2022			51.3
2023			51.3
Thereafter			507.1
Property, Plant and Equipment, Other Types [Member]
Operating Lease, Assets and Liabilities, Lessee [Abstract]
Operating Lease, Right-of-Use Asset, Gross		$ 48.0
Lease, Cost [Abstract]
Operating Lease, Cost	[2]	4.9	7.9	6.4
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
2020		3.4
2021		3.3
2022		3.4
2023		3.4
2024		3.5
Thereafter		51.3
Total minimum obligation		68.3
Interest component of obligation		(21.6)
Present value of minimum obligation		46.7
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019			5.2
2020			5.2
2021			5.1
2022			5.1
2023			5.1
Thereafter			56.3
Purchased Power Agreements
Operating Lease, Assets and Liabilities, Lessee [Abstract]
Operating Lease, Right-of-Use Asset, Gross		500.3
Lease, Cost [Abstract]
Operating Lease, Cost		48.1	51.1	$ 51.4
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
2020	[3],[4]	46.2
2021	[3],[4]	46.2
2022	[3],[4]	46.2
2023	[3],[4]	46.2
2024	[3],[4]	46.2
Thereafter	[3],[4]	404.5
Total minimum obligation	[3],[4]	635.5
Interest component of obligation	[3],[4]	(160.0)
Present value of minimum obligation	[3],[4]	$ 475.5
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019	[5],[6]		46.7
2020	[5],[6]		46.2
2021	[5],[6]		46.2
2022	[5],[6]		46.2
2023	[5],[6]		46.2
Thereafter	[5],[6]		$ 450.8

[1] PPA capacity payments are included in electric fuel and purchased power on the statements of income. Expense for other operating leases is included in O&M expense.

[2] Includes short-term lease expense of $1.5 million, $1.1 million and $1.2 million for 2019, 2018 and 2017, respectively.

[3] Amounts do not include PPAs accounted for as executory contracts and/or contingent payments, such as energy payments on renewable PPAs.

[4] PPA operating leases contractually expire at various dates through 2033.

[5] Amounts do not include PPAs accounted for as executory contracts and/or contingent payments, such as energy payments on renewable PPAs.

[6] PPA operating leases contractually expire at various dates through 2033.