Commitments and Contingencies, Asset Retirement Obligations (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019		Dec. 31, 2018
Asset Retirement Obligations [Line Items]
Amounts Incurred	[1]	$ (10.3)		$ 0.0
Amounts Settled		3.2	[2]	6.6	[3]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		2,177.9	[4]	2,083.9
Amounts Incurred	[1]	(10.3)		0.0
Accretion		108.4		103.2
Cash flow revisions		(13.1)	[5]	(2.6)	[6]
Ending balance		2,280.3		2,177.9	[4]
Legally restricted assets, for purposes of funding future nuclear decommissioning		2,400.0		2,100.0
Electric Plant Nuclear Production Decommissioning
Asset Retirement Obligations [Line Items]
Amounts Incurred	[1]	0.0
Amounts Settled		0.0	[2]	0.0	[3]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		1,968.3	[4]	1,873.6
Amounts Incurred	[1]	0.0
Accretion		99.5		94.7
Cash flow revisions		0.0	[5]	0.0	[6]
Ending balance		2,067.8		1,968.3	[4]
Electric Plant Wind Production
Asset Retirement Obligations [Line Items]
Amounts Incurred	[1]	(10.3)
Amounts Settled		0.0	[2]	0.0	[3]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		104.9	[4]	94.1
Amounts Incurred	[1]	(10.3)
Accretion		4.8		4.3
Cash flow revisions		(6.9)	[5]	6.5	[6]
Ending balance		113.1		104.9	[4]
Electric Plant Steam Production Ash Containment
Asset Retirement Obligations [Line Items]
Amounts Incurred	[2]	0.0
Amounts Settled		3.2	[2]	6.6	[3]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		49.2	[4]	64.0
Amounts Incurred	[2]	0.0
Accretion		1.8		2.1
Cash flow revisions		(1.0)	[5]	(10.3)	[6]
Ending balance		46.8		49.2	[4]
Electric Plant Electric Distribution
Asset Retirement Obligations [Line Items]
Amounts Incurred	[1]	0.0
Amounts Settled		0.0	[2]	0.0	[3]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		14.5	[4]	5.8
Amounts Incurred	[1]	0.0
Accretion		0.6		0.2
Cash flow revisions		0.0	[5]	8.5	[6]
Ending balance		15.1		14.5	[4]
Electric Plant Other
Asset Retirement Obligations [Line Items]
Amounts Incurred	[1]	0.0
Amounts Settled		0.0	[2]	0.0	[3]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		1.8	[4]	1.9
Amounts Incurred	[1]	0.0
Accretion		0.0		0.0
Cash flow revisions		(1.6)	[5]	(0.1)	[6]
Ending balance		0.2		1.8	[4]
Natural Gas Plant Gas Transmission and Distribution
Asset Retirement Obligations [Line Items]
Amounts Incurred	[1]	0.0
Amounts Settled		0.0	[2]	0.0	[3]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		38.2	[4]	43.6
Amounts Incurred	[1]	0.0
Accretion		1.6		1.8
Cash flow revisions		(3.6)	[5]	(7.2)	[6]
Ending balance		36.2		38.2	[4]
Natural Gas Plant Other
Asset Retirement Obligations [Line Items]
Amounts Incurred	[1]	0.0
Amounts Settled		0.0	[2]	0.0	[3]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		0.2	[4]	0.2
Amounts Incurred	[1]	0.0
Accretion		0.1		0.0
Cash flow revisions		0.0	[5]	0.0	[6]
Ending balance		0.3		0.2	[4]
Common and Other Property Common Miscellaneous
Asset Retirement Obligations [Line Items]
Amounts Incurred	[1]	0.0
Amounts Settled		0.0	[2]	0.0	[3]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		0.8	[4]	0.7
Amounts Incurred	[1]	0.0
Accretion		0.0		0.1
Cash flow revisions		0.0	[5]	0.0	[6]
Ending balance		0.8		0.8	[4]
Removal Costs [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Regulatory Liabilities		$ 520.3		$ 484.6

[1] Amounts incurred relate to the wind farms placed in service in 2019 (Lake Benton and Foxtail).

[2] Amounts settled related to closure of certain ash containment facilities.

[3] Amounts settled related to asbestos abatement projects and closure of certain ash containment facilities.

[4] There were no ARO amounts incurred in 2018.

[5] In 2019, AROs were revised for changes in timing and estimates of cash flows. Changes in wind AROs were driven by new dismantling studies. Changes in gas transmission and distribution AROs were primarily related to increased gas line mileage and number of services, which were more than offset by decreased inflation rates. Changes in steam and other production AROs primarily related to the cost estimates to remediate ponds at production facilities.

[6] In 2018, AROs were revised for changes in timing and estimates of cash flows. Changes in gas transmission and 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.