Regulatory Assets and Liabilities, Regulatory Liabilities (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 85.8	$ 68.8
Regulatory Liability, Noncurrent		780.9	784.6
Deferred Income Tax Adjustments and TCJA Refunds
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	[1]	2.2	0.0
Regulatory Liability, Noncurrent	[1]	$ 569.8	568.6
Regulatory liability, remaining amortization period		Various
Plant Removal Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 0.0	0.0
Regulatory Liability, Noncurrent		$ 187.7	196.9
Regulatory liability, remaining amortization period		Plant lives
Revenue Subject to Refund
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 11.3	6.8
Regulatory Liability, Noncurrent		$ 8.1	6.5
Regulatory liability, remaining amortization period, minimum		1 year
Regulatory liability, remaining amortization period, maximum		2 years
Gain From Asset Sales
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 0.0	0.0
Regulatory Liability, Noncurrent		$ 2.4	2.5
Regulatory liability, remaining amortization period		Various
Deferred Electric Energy Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 56.5	48.5
Regulatory Liability, Noncurrent		$ 0.0	0.0
Regulatory liability, remaining amortization period, maximum		1 year
Contract Valuation Adjustments
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	[2]	$ 14.7	12.7
Regulatory Liability, Noncurrent	[2]	$ 0.0	0.0
Regulatory liability, remaining amortization period, maximum		1 year
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 1.1	0.8
Regulatory Liability, Noncurrent		$ 12.9	$ 10.1
Regulatory liability, remaining amortization period		Various

[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.