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		$ 120.6	$ 77.3
Regulatory Asset, Noncurrent		1,010.7	950.3
Past expenditures not currently earning a return		$ 50.0	44.0
Pension and Retiree Medical Obligations
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Various
Regulatory Asset, Current		$ 26.1	28.0
Regulatory Asset, Noncurrent		559.0	565.3
Depreciation Differences
Regulatory Assets [Line Items]
Regulatory Asset, Current		17.5	19.8
Regulatory Asset, Noncurrent		$ 107.0	69.4
Regulatory Asset Amortization Period, minimum		1 year
Regulatory Asset Amortization Period, maximum		13 years
Recoverable Deferred Taxes on AFUDC Recorded in Plant
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Plant lives
Regulatory Asset, Current		$ 0.0	0.0
Regulatory Asset, Noncurrent		$ 101.9	87.0
Net AROs
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Plant lives
Regulatory Asset, Current	[1]	$ 0.0	0.0
Regulatory Asset, Noncurrent	[1]	$ 98.9	80.5
Excess deferred taxes - TCJA
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Various
Regulatory Asset, Current		$ 0.0	0.0
Regulatory Asset, Noncurrent		$ 62.0	53.9
Purchased Power Agreements
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Term of related contract
Regulatory Asset, Current		$ 1.7	1.3
Regulatory Asset, Noncurrent		$ 26.3	28.0
Property Tax
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Various
Regulatory Asset, Current		$ 5.6	0.0
Regulatory Asset, Noncurrent		9.8	16.1
Conservation Programs
Regulatory Assets [Line Items]
Regulatory Asset, Current	[2]	7.3	7.0
Regulatory Asset, Noncurrent	[2]	$ 6.5	5.5
Regulatory Asset Amortization Period, minimum		1 year
Regulatory Asset Amortization Period, maximum		2 years
Losses on Reacquired Debt
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Term of related debt
Regulatory Asset, Current		$ 1.2	1.2
Regulatory Asset, Noncurrent		3.7	4.9
Gas Pipeline Inspection Costs
Regulatory Assets [Line Items]
Regulatory Asset, Current		0.7	1.8
Regulatory Asset, Noncurrent		$ 3.1	7.8
Regulatory Asset Amortization Period, minimum		1 year
Regulatory Asset Amortization Period, maximum		2 years
Contract Valuation Adjustments
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Term of related contract
Regulatory Asset, Current	[3]	$ 2.6	6.0
Regulatory Asset, Noncurrent	[3]	$ 0.0	2.6
Recoverable Purchased Natural Gas and Electric Energy Costs
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Less than one year
Regulatory Asset, Current		$ 51.2	7.6
Regulatory Asset, Noncurrent		$ 0.0	0.0
Regulatory Asset Amortization Period, maximum		1 year
Other Regulatory Assets
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Various
Regulatory Asset, Current		$ 6.7	4.6
Regulatory Asset, Noncurrent		$ 32.5	$ 29.3

[1] Includes amounts recorded for future recovery of AROs.

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

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