Commitments and Contingencies, Asset Retirement Obligations (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019		Dec. 31, 2018
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		$ 32.4	[1]	$ 28.5
Amounts Incurred		16.0	[2]	0.0	[1]
Amounts Settled		(1.6)	[3]	0.0	[1]
Accretion		2.2		1.6
Cash flow revisions		28.3	[4]	2.3	[5]
Ending balance		77.3		32.4	[1]
Electric Plant Steam Production Asbestos
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		22.0	[1]	20.3
Amounts Incurred		0.0	[2]	0.0
Amounts Settled		(1.6)	[3]	0.0
Accretion		1.4		1.2
Cash flow revisions		29.5	[4]	0.5	[5]
Ending balance		51.3		22.0	[1]
Electric Plant Wind Production [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		0.0
Amounts Incurred	[2]	16.0
Amounts Settled	[3]	0.0
Accretion		0.4
Cash flow revisions	[4]	0.0
Ending balance		16.4		0.0
Electric Plant Distribution
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		9.1	[1]	7.0
Amounts Incurred		0.0	[2]	0.0
Amounts Settled		0.0	[3]	0.0
Accretion		0.4		0.3
Cash flow revisions		0.0	[4]	1.8	[5]
Ending balance		9.5		9.1	[1]
Electric Plant Other Sources
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		1.3	[1]	1.2
Amounts Incurred		0.0	[2]	0.0
Amounts Settled		0.0	[3]	0.0
Accretion		0.0		0.1
Cash flow revisions		(1.2)	[4]	0.0	[5]
Ending balance		$ 0.1		$ 1.3	[1]

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

[2] Amounts incurred related to the Hale wind farm placed in service in 2019.

[3] Amounts settled related to asbestos abatement projects.

[4] In 2019, AROs were revised for changes in timing and estimates of cash flows. Changes in steam production AROs primarily related to the cost estimates to remediate ponds at production facilities.

[5] In 2018, AROs were revised for changes in timing and estimates of cash flows. Changes in electric distribution AROs were primarily related to increased labor costs.