Regulatory Assets and Liabilities, Regulatory Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Regulatory Asset, Current	$ 22.6	$ 23.1
Regulatory Asset, Noncurrent	285.5	282.2
Environmental Remediation Costs
Regulatory Assets [Line Items]
Regulatory Asset, Current	16.0	16.0
Regulatory Asset, Noncurrent	$ 135.2	136.1
Regulatory asset, remaining amortization period	Various
Pension and Retiree Medical Obligations
Regulatory Assets [Line Items]
Regulatory Asset, Current	$ 5.5	5.7
Regulatory Asset, Noncurrent	$ 85.1	87.5
Regulatory asset, remaining amortization period	Various
Excess deferred taxes - TCJA
Regulatory Assets [Line Items]
Regulatory Asset, Current	$ 0.0	0.0
Regulatory Asset, Noncurrent	$ 25.2	22.6
Regulatory asset, remaining amortization period	Various
Recoverable Deferred Taxes on AFUDC Recorded in Plant
Regulatory Assets [Line Items]
Regulatory Asset, Current	$ 0.0	0.0
Regulatory Asset, Noncurrent	$ 16.9	14.3
Regulatory asset, remaining amortization period	Plant lives
State Commission Adjustments
Regulatory Assets [Line Items]
Regulatory Asset, Current	$ 0.8	0.7
Regulatory Asset, Noncurrent	$ 16.8	15.9
Regulatory asset, remaining amortization period	Plant lives
Losses on Reacquired Debt
Regulatory Assets [Line Items]
Regulatory Asset, Current	$ 0.2	0.7
Regulatory Asset, Noncurrent	$ 2.4	2.7
Regulatory asset, remaining amortization period	Term of related debt
Other Regulatory Assets
Regulatory Assets [Line Items]
Regulatory Asset, Current	$ 0.1	0.0
Regulatory Asset, Noncurrent	$ 3.9	$ 3.1
Regulatory asset, remaining amortization period	Various