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Fair Value Measurements Fair Value Measurements - Reconciliation of Level 3 Assets and Liabilities (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] | Accumulated other comprehensive (loss) |  |
Fair Market Value | $ 4,978.3 | $ 4,231.1 |
Fair Market Value | 1,488.7 | 1,964.4 |
Level 3 | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Change in unrealized investment gains (losses) | 1.3 | (15.7) |
Fair Market Value | 311.0 | 489.9 |
Level 3 | Fair Value, Recurring [Member] | Liability |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Asset reclassification | 0.0 | 0.0 |
Liabilities at beginning of year | (3.6) | (16.5) | $ (31.7)
Liability purchases | 0.0 | 0.0 |
Liability transfers | 0.0 | 0.0 |
Liability, Transfers out of Level 3 | 0.0 | 0.0 |
Liability sales | 0.0 | 0.0 |
Liability, increase (decrease) in fv included in net income | 12.9 | 15.2 |
Change in unrealized gains or losses | 12.9 | 15.2 |
Level 3 | Fair Value, Recurring [Member] | Loss Portfolio Transfer | Liability |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Asset reclassification | 0.0 | 0.0 |
Liabilities at beginning of year | (3.6) | (16.5) | (31.7)
Liability purchases | 0.0 | 0.0 |
Liability transfers | 0.0 | 0.0 |
Liability, Transfers out of Level 3 | 0.0 | 0.0 |
Liability sales | 0.0 | 0.0 |
Liability, increase (decrease) in fv included in net income | 12.9 | 15.2 |
Change in unrealized gains or losses | 12.9 | 15.2 |
Level 3 | Fair Value, Recurring [Member] | Commercial mortgage loans |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Change in unrealized investment gains (losses) | 0.0 | (17.9) |
Fair Market Value | 79.7 | 274.9 |
Level 3 | Fair Value, Recurring [Member] | Middle market loans and other private debt |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Change in unrealized investment gains (losses) | 0.0 | 0.5 |
Fair Market Value | 61.0 | 84.8 |
Level 3 | Fair Value, Recurring [Member] | Asset-backed |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Change in unrealized investment gains (losses) | 1.1 | 1.7 |
Fair Market Value | 127.6 | 82.9 |
Level 3 | Fair Value, Recurring [Member] | Global corporate securities |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Change in unrealized investment gains (losses) | 0.0 | (0.2) |
Fair Market Value | 18.5 | 14.4 |
Level 3 | Fair Value, Recurring [Member] | Short-term investments |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Change in unrealized investment gains (losses) | 0.0 | 0.0 |
Fair Market Value | 0.0 | 18.0 |
Level 3 | Fair Value, Recurring [Member] | Equity Securities |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Change in unrealized investment gains (losses) |  | 0.0 |
Fair Market Value |  | 0.0 |
Level 3 | Fair Value, Recurring [Member] | Asset backed securities, privately-held afs |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Fair Market Value | 24.2 | 14.9 |
Change in unrealized investment gains (losses) | 0.2 | 0.2 |
Assets | Level 3 | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Purchases and issuances | 71.4 | 113.7 |
Transfers Into Level 3 | 0.0 | 5.3 |
Transfers out of Level 3 | 0.0 | 12.1 |
Asset reclassification | 0.0 | 0.0 |
Settlements and sales | (239.7) | (137.2) |
Increase (decrease) in fair value included in net income | (10.6) | (12.8) |
Fair Market Value |  | 489.9 | 533.0
Assets | Level 3 | Fair Value, Recurring [Member] | Commercial mortgage loans |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Purchases and issuances | 0.5 | 40.6 |
Transfers Into Level 3 | 0.0 | 0.0 |
Transfers out of Level 3 | 0.0 | 0.0 |
Asset reclassification | 0.0 | 0.0 |
Settlements and sales | (184.4) | (61.5) |
Increase (decrease) in fair value included in net income | (11.3) | (16.3) |
Fair Market Value |  | 274.9 | 312.1
Assets | Level 3 | Fair Value, Recurring [Member] | Middle market loans and other private debt |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Purchases and issuances | 0.4 | 18.3 |
Transfers Into Level 3 | 0.0 | 0.0 |
Transfers out of Level 3 | 0.0 | 0.0 |
Asset reclassification | 10.9 | 0.0 |
Settlements and sales | (35.5) | (41.9) |
Increase (decrease) in fair value included in net income | 0.4 | 1.5 |
Fair Market Value |  | 84.8 | 106.9
Assets | Level 3 | Fair Value, Recurring [Member] | Asset-backed |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Purchases and issuances | 56.1 | 21.9 |
Transfers Into Level 3 | 0.0 | 5.3 |
Transfers out of Level 3 | 0.0 | (5.5) |
Asset reclassification | 0.0 | 0.0 |
Settlements and sales | (12.4) | (7.6) |
Increase (decrease) in fair value included in net income | 1.0 | 2.0 |
Fair Market Value |  | 82.9 | 66.8
Assets | Level 3 | Fair Value, Recurring [Member] | Global corporate securities |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Purchases and issuances | 4.4 | 0.0 |
Transfers Into Level 3 | 0.0 | 0.0 |
Transfers out of Level 3 | 0.0 | 0.0 |
Asset reclassification | 0.0 | 0.0 |
Settlements and sales | (0.3) | (0.4) |
Increase (decrease) in fair value included in net income | 0.0 | (0.2) |
Fair Market Value |  | 14.4 | 15.0
Assets | Level 3 | Fair Value, Recurring [Member] | Short-term investments |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Purchases and issuances | 0.0 | 18.2 |
Transfers Into Level 3 | 0.0 | 0.0 |
Transfers out of Level 3 | 0.0 | 0.0 |
Asset reclassification | (10.9) | 0.0 |
Settlements and sales | (7.1) | (25.8) |
Increase (decrease) in fair value included in net income | 0.0 | 0.0 |
Fair Market Value |  | 18.0 | 25.6
Assets | Level 3 | Fair Value, Recurring [Member] | Equity Securities |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Purchases and issuances |  | 0.0 |
Transfers Into Level 3 |  | 0.0 |
Transfers out of Level 3 |  | (6.6) |
Asset reclassification |  | 0.0 |
Settlements and sales |  | 0.0 |
Increase (decrease) in fair value included in net income |  | 0.0 |
Fair Market Value |  |  | 6.6
Assets | Level 3 | Fair Value, Recurring [Member] | Asset backed securities, privately-held afs |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Fair Market Value |  | 14.9 | $ 0.0
Purchases and issuances | 10.0 | 14.7 |
Transfers Into Level 3 | 0.0 | 0.0 |
Transfers out of Level 3 | 0.0 | 0.0 |
Asset reclassification | 0.0 | 0.0 |
Settlements and sales | 0.0 | 0.0 |
Increase (decrease) in fair value included in net income | $ (0.7) | $ 0.2 |

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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 103 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-103 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 100 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-100 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(2)(i) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 7: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (aa) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-2 Reference 8: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481830/320-10-45-1 Reference 9: http://www.xbrl.org/2003/role/disclosureRef -Topic 942 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(6)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1 Reference 10: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479130/326-30-45-1 + Details
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- Definition Indicates line item in statement of income or comprehensive income that includes unrealized gain (loss) from asset measured at fair value using unobservable input (level 3) and still held. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-3 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 102 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-102 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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- Definition Amount of unrealized gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3) and still held. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 101 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-101 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-3 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 102 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-102 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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- Definition Amount of unrealized gain (loss) recognized in income from liability measured at fair value on recurring basis using unobservable input (level 3) and still held. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-3 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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- Definition Amount of transfers of financial instrument classified as a liability into level 3 of the fair value hierarchy. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-3 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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- Definition Amount of transfers of financial instrument classified as a liability out of level 3 of the fair value hierarchy. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-3 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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- Definition Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3). + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 101 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-101 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-3 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 102 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-102 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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- Definition Amount of issuances of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 101 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-101 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-3 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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- Definition Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 101 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-101 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-3 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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- Definition Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 101 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-101 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-3 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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- Definition Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 101 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-101 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-3 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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- Definition Amount of gain (loss) recognized in income from liability measured at fair value on recurring basis using unobservable input (level 3). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-3 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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- Definition Amount of purchases of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-3 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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