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Pension and Other Postretirement Benefit Plans - Recurring Fair Value Measures, Activity in Level 3 (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022
Pension |  |
Activity in Level 3 [Roll Forward] |  |
Fair Value, Beginning of Year | $ 568.6 | $ 745.7
Fair Value, End of Year | 589.0 | 568.6
Pension | Level 3 |  |
Activity in Level 3 [Roll Forward] |  |
Fair Value, Beginning of Year | 22.4 |
Fair Value, End of Year | 18.9 | 22.4
Pension | Real Estate |  |
Activity in Level 3 [Roll Forward] |  |
Fair Value, Beginning of Year | 22.4 |
Fair Value, End of Year | 18.9 | 22.4
Pension | Real Estate | Level 3 |  |
Activity in Level 3 [Roll Forward] |  |
Fair Value, Beginning of Year | 22.4 | 21.6
Actual Return on Plan Assets | (3.1) | 1.0
Purchases, Sales, and Settlements – Net | (0.4) | (0.2)
Fair Value, End of Year | 18.9 | 22.4
Postretirement Health and Life |  |
Activity in Level 3 [Roll Forward] |  |
Fair Value, Beginning of Year | 162.6 | 201.8
Fair Value, End of Year | 173.8 | 162.6
Postretirement Health and Life | Level 3 |  |
Activity in Level 3 [Roll Forward] |  |
Fair Value, Beginning of Year | 0.0 |
Fair Value, End of Year | 0.0 | 0.0
Postretirement Health and Life | Private Equity | Level 3 |  |
Activity in Level 3 [Roll Forward] |  |
Fair Value, Beginning of Year | $ 0.0 | 2.0
Actual Return on Plan Assets |  | (1.5)
Purchases, Sales, and Settlements – Net |  | (0.5)
Fair Value, End of Year |  | $ 0.0

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- Definition Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 17 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480482/715-20-55-17 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(02) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 6: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 7: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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