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Operations and Significant Accounting Policies - Other Income (Expense) - Other (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
Other Income (Expense) - Other [Line Items] |  |  |  |
Total Other Income (Expense) - Other |  | $ 22.4 | $ 8.7 | $ 14.7
Pension and Other Postretirement Benefit Plans Non-Service Credit [Member] |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Total Other Income (Expense) - Other | [1] | 9.8 | 6.1 | 8.6
Interest and Investment Earnings [Member] |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Total Other Income (Expense) - Other |  | 0.0 | 2.3 | 1.6
AFUDC - Equity [Member] |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Total Other Income (Expense) - Other |  | 2.7 | 2.6 | 1.9
Gain on Land Sales [Member] |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Total Other Income (Expense) - Other |  | 0.0 | 0.1 | 0.4
PSA Liability |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Total Other Income (Expense) - Other | [2] | 10.2 | 0.0 | 0.0
Other Income (Expense) - Other [Member] |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Total Other Income (Expense) - Other |  | $ (0.3) | $ (2.4) | $ 2.2
[1] | These are components of net periodic pension and other postretirement benefit cost other than service cost. (See Note 12. Pension and Other Postretirement Benefit Plans.)
[2] | The gain on removal of the PSA liability for the Northern Wind project upon decommissioning of the legacy wind energy facility assets, which was more than offset by a reserve for an anticipated loss on the sale of the Northern Wind project, was recorded in Cost of Sales - Non-Utility on the Consolidated Statement of Income.

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- Definition [Line Items] for Other Income (Expense) - Other [Table] + References No definition available. + Details
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- Definition Amount of income (expense) related to nonoperating activities, classified as other. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.9) -URI https://asc.fasb.org/extlink&oid=126953954&loc=SL114868664-224227 + Details
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