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		$ 20.9	$ 20.7
Regulatory Liability, Noncurrent		400.1	386.8
Deferred Income Tax Adjustments and TCJA Refunds
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	[1]	0.2	0.0
Regulatory Liability, Noncurrent	[1]	$ 238.3	236.1
Regulatory liability, remaining amortization period		Various
Plant Removal Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 0.0	0.0
Regulatory Liability, Noncurrent		$ 157.7	146.4
Regulatory liability, remaining amortization period		Plant lives
Deferred Electric Production And Natural Gas Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 13.4	13.9
Regulatory Liability, Noncurrent		$ 0.0	0.0
Regulatory liability remaining amortization period, maximum		Less than one year
DOE Settlement
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 6.2	5.3
Regulatory Liability, Noncurrent		$ 0.0	0.0
Regulatory liability remaining amortization period, maximum		Less than one year
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 1.1	1.5
Regulatory Liability, Noncurrent		$ 4.1	$ 4.3
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.