Regulatory Assets and Liabilities, Regulatory Liabilities (Details) - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 118.1	$ 85.8
Regulatory Liability, Noncurrent		732.3	780.9
Regulatory assets not currently earning a return		56.5	50.5
Other Current Liabilities
Regulatory Liabilities [Line Items]
Entity's Recorded Provision for Revenue Subject To Refund		3.9	0.0
Deferred Income Tax Adjustments and TCJA Refunds
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	[1]	6.9	2.2
Regulatory Liability, Noncurrent	[1]	534.9	569.8
Plant Removal Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		0.0	0.0
Regulatory Liability, Noncurrent		174.5	187.7
Revenue Subject to Refund
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		14.6	11.3
Regulatory Liability, Noncurrent		1.1	8.1
Gain From Asset Sales
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		0.0	0.0
Regulatory Liability, Noncurrent		2.4	2.4
Deferred Electric Energy Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		81.6	56.5
Regulatory Liability, Noncurrent		0.0	0.0
Contract Valuation Adjustments
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	[2]	11.7	14.7
Regulatory Liability, Noncurrent	[2]	0.0	0.0
Other
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		3.3	1.1
Regulatory Liability, Noncurrent		$ 19.4	$ 12.9

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

[2] Includes costs for conservation programs, as well as incentives allowed in certain jurisdictions.