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Regulatory Assets and Liabilities - Regulatory Assets (Details) - USD ($) $ in Millions | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Sep. 30, 2021 | Jan. 01, 2020
Regulatory assets |  |  |  |
Other current assets | $ 24.9 | $ 42.3 |  |
Regulatory assets | 3,249.8 | 3,264.6 |  |
Total regulatory assets | 3,274.7 | 3,306.9 |  |
Allowance for return on equity capitalized for regulatory purposes | 26.7 | 27.3 |  |
Regulatory assets not earning a return | 254.6 |  |  |
Regulatory assets earning a return based on short-term interest rates | 5.4 |  |  |
Regulatory assets earning a return based on long-term interest rates | 129.7 |  |  |
Regulatory assets earning a return based on return on equity rates | 2.5 |  |  |
Estimated future cash expenditures for environmental remediation | 463.7 | 499.6 |  |
Pension and OPEB costs |  |  |  |
Regulatory assets |  |  |  |
Total regulatory assets | 731.7 | 714.3 |  |
Plant retirement related items |  |  |  |
Regulatory assets |  |  |  |
Total regulatory assets | 646.2 | 688.6 |  |
Environmental remediation costs |  |  |  |
Regulatory assets |  |  |  |
Total regulatory assets | 596.8 | 610.7 |  |
Cash expenditures for environmental remediation costs | 133.1 |  |  |
Estimated future cash expenditures for environmental remediation | 463.7 |  |  |
Income tax related items |  |  |  |
Regulatory assets |  |  |  |
Total regulatory assets | 449.9 | 461.9 |  |
Asset retirement obligations (AROs) |  |  |  |
Regulatory assets |  |  |  |
Total regulatory assets | 162.0 | 169.7 |  |
Derivatives |  |  |  |
Regulatory assets |  |  |  |
Total regulatory assets | 130.3 | 133.8 |  |
Uncollectible expense |  |  |  |
Regulatory assets |  |  |  |
Total regulatory assets | 127.7 | 69.3 |  |
System support resource (SSR) |  |  |  |
Regulatory assets |  |  |  |
Total regulatory assets | 113.2 | 123.5 |  |
Recovery period of regulatory asset |  |  |  | 15 years
Securitization |  |  |  |
Regulatory assets |  |  |  |
Total regulatory assets | 85.9 | 92.4 |  |
Bluewater |  |  |  |
Regulatory assets |  |  |  |
Total regulatory assets | 45.3 | 20.9 |  |
Energy efficiency programs |  |  |  |
Regulatory assets |  |  |  |
Total regulatory assets | 33.9 | 33.9 |  |
Energy costs recoverable through rate adjustments |  |  |  |
Regulatory assets |  |  |  |
Total regulatory assets | 3.2 | 26.9 |  |
MERC extraordinary natural gas costs |  |  |  |
Regulatory assets |  |  |  |
Total regulatory assets | 0.8 | 35.1 |  |
Recovery period of regulatory asset |  |  | 27 months |
Other, net |  |  |  |
Regulatory assets |  |  |  |
Total regulatory assets | $ 147.8 | $ 125.9 |  |

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- Definition Carrying value of the obligation (known or estimated) arising from requirements to perform activities to remediate one or more sites, payable after twelve months or beyond the next operating cycle if longer. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 450 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org//1943274/2147483076/450-20-50-1 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 410 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481931/410-30-50-7 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 410 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 10 -Subparagraph (d)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481931/410-30-50-10 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.24) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1 + Details
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- Definition Amortization period for the recovery of regulatory asset, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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- Definition Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 980 -SubTopic 340 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481878/980-340-50-1 + Details
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- Definition Remaining balance of regulatory assets for which no return on investment during recovery is provided. In some cases, a regulator may permit an enterprise to include a cost that would be charged to expense by an unregulated enterprise as an allowable cost over a period of time by amortizing that cost for rate-making purposes, but the regulator does not include the unrecovered amount in the rate base. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 980 -SubTopic 340 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481878/980-340-50-1 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 55 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481849/980-340-55-2 + Details
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