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]
Short-term Lease, Cost		$ 1.4	$ 2.0	$ 2.7
Operating Lease, Weighted Average Discount Rate, Percent		3.80%
Operating Lease, Assets and Liabilities, Lessee [Abstract]
Operating Lease, Right-of-Use Asset, Gross		$ 628.5
Operating Lease, Right-of-Use Asset, Accumulated Depreciation		(64.7)
Operating Lease, Right-of-Use Asset		563.8	0.0
Lease, Cost [Abstract]
Operating Lease, Cost	[1]	85.0	76.2	76.9
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
2020		101.4
2021		102.9
2022		108.3
2023		104.9
2024		106.3
Thereafter		164.7
Total minimum obligation		688.5
Interest component of obligation		(82.9)
Present value of minimum obligation		605.6
Less current portion		(79.9)	0.0
Operating Lease, Liability, Noncurrent		$ 525.7	0.0
Weighted-average remaining lease term in years		6 years 8 months 12 days
Operating Leases, Future Minimum Payments Due [Abstract]
2019			78.5
2021			75.5
2022			76.3
2020			74.5
2023			76.6
Thereafter			179.5
Property, Plant and Equipment, Other Types [Member]
Operating Lease, Assets and Liabilities, Lessee [Abstract]
Operating Lease, Right-of-Use Asset, Gross		$ 72.2
Lease, Cost [Abstract]
Operating Lease, Cost	[2]	9.1	13.7	14.2
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
2020		7.9
2021		8.0
2022		11.9
2023		7.0
2024		6.8
Thereafter		44.9
Total minimum obligation		86.5
Interest component of obligation		(16.7)
Present value of minimum obligation		69.8
Office Space and Other Equipment
Operating Leases, Future Minimum Payments Due [Abstract]
2019	[3],[4]		65.0
2021	[3],[4]		67.1
2022	[3],[4]		68.2
2020	[3],[4]		66.1
2023	[3],[4]		69.3
Thereafter	[3],[4]		143.5
Purchased Power Agreements
Operating Lease, Assets and Liabilities, Lessee [Abstract]
Operating Lease, Right-of-Use Asset, Gross		556.3
Lease, Cost [Abstract]
Operating Lease, Cost		75.9	62.5	$ 62.7
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
2020	[5],[6]	93.5
2021	[5],[6]	94.9
2022	[5],[6]	96.4
2023	[5],[6]	97.9
2024	[5],[6]	99.5
Thereafter	[5],[6]	119.8
Total minimum obligation	[5],[6]	602.0
Interest component of obligation	[5],[6]	(66.2)
Present value of minimum obligation	[5],[6]	$ 535.8
Operating Leases, Future Minimum Payments Due [Abstract]
2019			13.5
2021			8.4
2022			8.1
2020			8.4
2023			7.3
Thereafter			$ 36.0

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

[2] Includes short-term lease expense of $1.4 million, $2.0 million and $2.7 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 2026.

[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 2026.