Commitments and Contingencies, Asset Retirement Obligations (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019		Dec. 31, 2018
Asset Retirement Obligations [Line Items]
Asset Retirement Obligation, Liabilities Settled		$ 0.0	[1]	$ 0.0	[2]
Asset Retirement Obligation, Liabilities Incurred		0.0	[1]	0.0	[2]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	[4]	17.9	[2],[3]	14.4
Accretion	[3]	0.7		0.5
Cash Flow Revisions		(0.9)	[3]	3.0	[4],[5]
Ending balance	[3]	17.7	[1]	17.9	[2],[4]
Electric Plant Electric Distribution
Asset Retirement Obligations [Line Items]
Asset Retirement Obligation, Liabilities Settled		0.0		0.0
Asset Retirement Obligation, Liabilities Incurred		0.0		0.0
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		4.6	[2]	0.0
Accretion		0.2		0.0
Cash Flow Revisions		0.0		4.6	[5]
Ending balance		4.8	[1]	4.6	[2]
Electric Plant Steam Production Asbestos
Asset Retirement Obligations [Line Items]
Asset Retirement Obligation, Liabilities Settled		0.0		0.0
Asset Retirement Obligation, Liabilities Incurred		0.0		0.0
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		3.8	[2]	3.7
Accretion		0.1		0.1
Cash Flow Revisions		0.4		0.0	[5]
Ending balance		4.3	[1]	3.8	[2]
Electric Plant Other
Asset Retirement Obligations [Line Items]
Asset Retirement Obligation, Liabilities Settled		0.0		0.0
Asset Retirement Obligation, Liabilities Incurred		0.0		0.0
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		0.4	[2]	0.4
Accretion		0.0		0.0
Cash Flow Revisions		(0.4)		0.0	[5]
Ending balance		0.0	[1]	0.4	[2]
Natural Gas Plant Gas Distribution
Asset Retirement Obligations [Line Items]
Asset Retirement Obligation, Liabilities Settled		0.0		0.0
Asset Retirement Obligation, Liabilities Incurred		0.0		0.0
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		9.1	[2]	10.3
Accretion		0.4		0.4
Cash Flow Revisions		(0.9)		(1.6)	[5]
Ending balance		$ 8.6	[1]	$ 9.1	[2]

[1] There were no ARO amounts incurred or settled in 2019.

[2] There were no ARO amounts incurred or settled in 2018.

[3] Included in other long-term liabilities balance in the consolidated balance sheet.

[4] Included in other long-term liabilities balance in the consolidated balance sheet.

[5] In 2018, AROs were revised for changes in timing and estimates of cash flows. Changes in gas distribution AROs were mainly related to increased gas line mileage and number of services, which were more than offset by increased discount rates. Changes in electric distribution AROs primarily related to increased labor costs.