Regulatory Assets and Liabilities, Regulatory Liabilities (Details) - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	[1]	$ 141.0	$ 262.4
Regulatory Liability, Noncurrent	[1]	1,937.1	1,984.7
Regulatory assets not currently earning a return		235.1	190.2
Deferred income tax adjustments and TCJA refunds
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	[2]	12.8	153.7
Regulatory Liability, Noncurrent	[2]	1,388.9	1,465.1
Plant Removal Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		0.0	0.0
Regulatory Liability, Noncurrent		520.3	484.6
Investment Tax Credit Deferrals
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	[3]	0.0	0.0
Regulatory Liability, Noncurrent	[3]	8.3	8.9
DOE Settlement
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		27.0	13.0
Regulatory Liability, Noncurrent		0.0	0.0
Deferred Electric Energy Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		24.2	22.8
Regulatory Liability, Noncurrent		0.0	0.0
Contract Valuation Adjustments
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	[4]	7.8	10.4
Regulatory Liability, Noncurrent	[4]	0.0	0.0
Renewable Resources and Environmental Initiatives
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		0.0	8.8
Regulatory Liability, Noncurrent		0.0	0.0
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		69.2	53.7
Regulatory Liability, Noncurrent		19.6	26.1
Other Current Liabilities
Regulatory Liabilities [Line Items]
Entity's Recorded Provision for Revenue Subject To Refund		$ 23.8	$ 12.5

[1] Revenue subject for refund of $23.8 million and $12.5 million for 2019 and 2018, respectively, is included in other current liabilities.

[2] Includes the revaluation of recoverable/regulated plant ADIT and revaluation impact of non-plant ADIT due to the TCJA.

[3] Includes impact of lower federal tax rate due to the TCJA.

[4] Includes the fair value of certain long-term PPAs used to meet energy capacity requirements and valuation adjustments on natural gas commodity purchases.