Regulatory Assets and Liabilities, Regulatory Assets (Details) - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 320.1	$ 280.3
Regulatory Asset, Noncurrent		1,125.0	1,454.1
Pension and Retiree Medical Obligations
Regulatory Assets [Line Items]
Regulatory Asset, Current		27.5	28.1
Regulatory Asset, Noncurrent		377.7	424.3
Excess deferred taxes - TCJA
Regulatory Assets [Line Items]
Regulatory Asset, Current		10.4	0.0
Regulatory Asset, Noncurrent		132.2	153.3
Net AROs
Regulatory Assets [Line Items]
Regulatory Asset, Current	[1]	0.0	0.0
Regulatory Asset, Noncurrent	[1]	117.9	323.4
Recoverable Deferred Taxes on AFUDC Recorded in Plant
Regulatory Assets [Line Items]
Regulatory Asset, Current		0.0	0.0
Regulatory Asset, Noncurrent		114.8	117.6
Benson purchase power agreement termination and asset purchase
Regulatory Assets [Line Items]
Regulatory Asset, Current		9.4	9.8
Regulatory Asset, Noncurrent		72.7	85.8
Contract Valuation Adjustments
Regulatory Assets [Line Items]
Regulatory Asset, Current	[2]	15.5	14.1
Regulatory Asset, Noncurrent	[2]	62.1	76.0
Laurentian biomass PPA termination
Regulatory Assets [Line Items]
Regulatory Asset, Current		19.2	18.1
Regulatory Asset, Noncurrent		53.9	73.3
PI extended power update
Regulatory Assets [Line Items]
Regulatory Asset, Current		3.1	3.1
Regulatory Asset, Noncurrent		52.6	55.8
Purchased Power Agreements
Regulatory Assets [Line Items]
Regulatory Asset, Current		2.7	2.8
Regulatory Asset, Noncurrent		36.5	36.6
Nuclear Refueling Outage Costs
Regulatory Assets [Line Items]
Regulatory Asset, Current		43.3	36.3
Regulatory Asset, Noncurrent		16.9	13.5
Sales True-Up and Revenue Decoupling
Regulatory Assets [Line Items]
Regulatory Asset, Current		53.8	38.3
Regulatory Asset, Noncurrent		16.3	6.7
Loss on Reacquired Debt
Regulatory Assets [Line Items]
Regulatory Asset, Current		1.7	2.1
Regulatory Asset, Noncurrent		13.8	15.5
Conservation Programs
Regulatory Assets [Line Items]
Regulatory Asset, Current	[3]	18.1	34.5
Regulatory Asset, Noncurrent	[3]	13.5	21.1
Environmental Remediation Costs
Regulatory Assets [Line Items]
Regulatory Asset, Current		1.3	1.3
Regulatory Asset, Noncurrent		11.8	14.3
Deferred Purchased Natural Gas and Electric Energy Costs
Regulatory Assets [Line Items]
Regulatory Asset, Current		6.2	5.6
Regulatory Asset, Noncurrent		5.7	12.6
State Commission Adjustments
Regulatory Assets [Line Items]
Regulatory Asset, Current		0.0	0.0
Regulatory Asset, Noncurrent		3.4	3.4
Renewable Resources and Environmental Initiatives
Regulatory Assets [Line Items]
Regulatory Asset, Current		72.2	39.2
Regulatory Asset, Noncurrent		0.6	0.4
Gas Pipeline Inspection and Remediation Costs
Regulatory Assets [Line Items]
Regulatory Asset, Current		26.2	27.4
Regulatory Asset, Noncurrent		0.0	0.0
Other
Regulatory Assets [Line Items]
Regulatory Asset, Current		9.5	19.6
Regulatory Asset, Noncurrent		$ 22.6	$ 20.5

[1] Includes amounts recorded for future recovery of AROs, less amounts recovered through nuclear decommissioning accruals and gains from decommissioning investments.

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

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