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Property, Plant and Equipment (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Plant and Equipment Associated with Schahfer Generating Station Retirement		$ 903.8
Depreciation	$ 672.1	655.6	$ 612.2
Regulatory Deferral - Joint Venture	5.3
Capitalized Computer Software, Amortization	49.4	56.7	55.5
Capitalized Computer Software, Gross	181.8	136.4
Capitalized Cloud Computing, Amortization	10.0	3.4	$ 1.6
Capitalized Cloud Computing, Gross	42.4	$ 12.7
Units 14 and 15
Property, Plant and Equipment [Line Items]
Plant and Equipment Associated with Schahfer Generating Station Retirement	600.0
Units Not Retired
Property, Plant and Equipment [Line Items]
Plant and Equipment Associated with Schahfer Generating Station Retirement	$ 201.5

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Amount of expense for amortization of capitalized computer software costs.

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Amount before accumulated amortization of capitalized costs for computer software, including but not limited to, acquired and internally developed computer software.

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