Commitments and Contingencies Leases (Details) - USD ($) $ in Millions		3 Months Ended	6 Months Ended	9 Months Ended
		Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2019	Dec. 31, 2018
Lessee, Lease, Description [Line Items]
Operating Lease, Weighted Average Discount Rate, Percent		4.10%		4.10%
Maximum lease term for short-term lease classification			12 months
Operating Lease, Assets and Liabilities, Lessee [Abstract]
Operating Lease, Right-of-Use Asset, Gross		$ 653.8		$ 653.8
Operating Lease, Right-of-Use Asset, Accumulated Depreciation		(59.4)		(59.4)
Operating Lease, Right-of-Use Asset		594.4		594.4	$ 0.0
Finance Lease, Assets and Liabilities, Lessee [Abstract]
Finance Lease, Right-of-Use Asset, Gross		221.2		221.2
Finance Lease, Right-of-Use Asset, Accumulated Depreciation		(80.8)		(80.8)
Finance Lease, Right-of-Use Asset		140.4		140.4
Operating Lease, Cost [Abstract]
Operating Lease, Cost	[1]	28.9		85.1
Payments of Financing Costs [Abstract]
Finance Lease, Right-of-Use Asset, Amortization		1.6		4.6
Finance Lease, Interest Expense		4.7		14.2
Finance Lease, Cost		6.3		18.8
Short-term Lease, Cost		0.3		1.0
Operating Leases after Adoption of ASC Topic 842
2019		27.2		27.2
2020		109.1		109.1
2021		109.0		109.0
2022		94.2		94.2
2023		80.9		80.9
Thereafter		317.8		317.8
Total minimum obligation		738.2		738.2
Interest component of obligation		(113.7)		(113.7)
Present value of minimum obligation		624.5		624.5
Less current portion		(85.0)		(85.0)
Operating lease liabilities		$ 539.5		$ 539.5	0.0
Weighted-average remaining lease term in years		8 years 1 month		8 years 1 month
Finance Leases after Adoption of ASC Topic 842
2019		$ 6.2		$ 6.2
2020		24.8		24.8
2021		23.6		23.6
2022		20.5		20.5
2023		20.3		20.3
Thereafter		419.8		419.8
Total minimum obligation		515.2		515.2
Interest component of obligation		(374.8)		(374.8)
Present value of minimum obligation		140.4		140.4
Less current portion		(6.7)		(6.7)
Noncurrent operating and finance lease liabilities		$ 133.7		$ 133.7
Weighted-average remaining lease term in years		38 years 11 months		38 years 11 months
Operating Leases before Adoption of ASC Topic 842
2019					106.3
2020					106.6
2021					105.9
2022					91.0
2023					78.1
Thereafter					342.0
Finance Leases before Adoption of ASC Topic 842
2019					24.9
2020					24.8
2021					23.6
2022					20.5
2023					20.3
Thereafter					420.4
Total minimum obligation					534.5
Interest component of obligation					(389.5)
Present value of minimum obligation					145.0
WYCO
Operating Lease, Assets and Liabilities, Lessee [Abstract]
Equity Method Investment, Ownership Percentage		50.00%		50.00%
Gas Storage Facilities [Member]
Finance Lease, Assets and Liabilities, Lessee [Abstract]
Finance Lease, Right-of-Use Asset, Gross		$ 200.5		$ 200.5
Purchased Power Agreements [Member]
Operating Lease, Assets and Liabilities, Lessee [Abstract]
Operating Lease, Right-of-Use Asset, Gross		585.1		585.1
Operating Lease, Cost [Abstract]
Operating Lease, Cost		24.6		73.6
Operating Leases after Adoption of ASC Topic 842
2019	[2],[3]	23.9		23.9
2020	[2],[3]	95.9		95.9
2021	[2],[3]	96.4		96.4
2022	[2],[3]	82.6		82.6
2023	[2],[3]	70.0		70.0
Thereafter	[2],[3]	288.6		288.6
Total minimum obligation	[2],[3]	657.4		657.4
Interest component of obligation	[2],[3]	(100.6)		(100.6)
Present value of minimum obligation	[2],[3]	556.8		556.8
Operating Leases before Adoption of ASC Topic 842
2019	[4],[5]				95.5
2020	[4],[5]				95.9
2021	[4],[5]				96.4
2022	[4],[5]				82.6
2023	[4],[5]				70.0
Thereafter	[4],[5]				288.6
Property, Plant and Equipment, Other Types [Member]
Operating Lease, Assets and Liabilities, Lessee [Abstract]
Operating Lease, Right-of-Use Asset, Gross		68.7		68.7
Operating Lease, Cost [Abstract]
Operating Lease, Cost	[6]	4.3		11.5
Operating Leases after Adoption of ASC Topic 842
2019		3.3		3.3
2020		13.2		13.2
2021		12.6		12.6
2022		11.6		11.6
2023		10.9		10.9
Thereafter		29.2		29.2
Total minimum obligation		80.8		80.8
Interest component of obligation		(13.1)		(13.1)
Present value of minimum obligation		67.7		67.7
Operating Leases before Adoption of ASC Topic 842
2019					10.8
2020					10.7
2021					9.5
2022					8.4
2023					8.1
Thereafter					$ 53.4
Pipelines [Member]
Finance Lease, Assets and Liabilities, Lessee [Abstract]
Finance Lease, Right-of-Use Asset, Gross		$ 20.7		$ 20.7

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

[3] PPA operating leases contractually expire at various dates through 2032.

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

[5] PPA operating leases contractually expire at various dates through 2032.

[6] Includes short-term lease expense of $0.3 million for the three months ended Sept. 30, 2019 and $1.0 million for the nine months ended Sept. 30, 2019.