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Fair Value Measurements Fair Value Measurements - Reconciliation of Level 3 Assets and Liabilities (Details) - USD ($) | 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 | $ 4,978,300,000 | $ 4,231,100,000 |
Fair Market Value | 1,488,700,000 | 1,964,400,000 |
Level 3 | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Change in unrealized investment gains (losses) | 1,300,000 | (15,700,000) |
Fair Market Value | 311,000,000.0 | 489,900,000 |
Level 3 | Fair Value, Recurring [Member] | Liability |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Asset reclassification | 0 | 0 |
Liabilities at beginning of year | (3,600,000) | (16,500,000) | $ (31,700,000)
Liability purchases | 0 | 0 |
Liability transfers | 0 | 0 |
Liability, Transfers out of Level 3 | 0 | 0 |
Liability sales | 0 | 0 |
Liability, increase (decrease) in fv included in net income | 12,900,000 | 15,200,000 |
Change in unrealized gains or losses | 12,900,000 | 15,200,000 |
Level 3 | Fair Value, Recurring [Member] | Loss Portfolio Transfer | Liability |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Asset reclassification | 0 | 0 |
Liabilities at beginning of year | (3,600,000) | (16,500,000) | (31,700,000)
Liability purchases | 0 | 0 |
Liability transfers | 0 | 0 |
Liability, Transfers out of Level 3 | 0 | 0 |
Liability sales | 0 | 0 |
Liability, increase (decrease) in fv included in net income | 12,900,000 | 15,200,000 |
Change in unrealized gains or losses | 12,900,000 | 15,200,000 |
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 | (17,900,000) |
Fair Market Value | 79,700,000 | 274,900,000 |
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 | 500,000 |
Fair Market Value | 61,000,000.0 | 84,800,000 |
Level 3 | Fair Value, Recurring [Member] | Asset-backed |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Change in unrealized investment gains (losses) | 1,100,000 | 1,700,000 |
Fair Market Value | 127,600,000 | 82,900,000 |
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 | (200,000) |
Fair Market Value | 18,500,000 | 14,400,000 |
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 |
Fair Market Value | 0 | 18,000,000.0 |
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 |
Fair Market Value |  | 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 | 24,200,000 | 14,900,000 |
Change in unrealized investment gains (losses) | 200,000 | 200,000 |
Assets | Level 3 | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Purchases and issuances | 71,400,000 | 113,700,000 |
Transfers Into Level 3 | 0 | 5,300,000 |
Transfers out of Level 3 |  | 12,100,000 |
Asset reclassification | 0 | 0 |
Settlements and sales | (239,700,000) | (137,200,000) |
Increase (decrease) in fair value included in net income | (10,600,000) | (12,800,000) |
Fair Market Value |  | 489,900,000 | 533,000,000.0
Assets | Level 3 | Fair Value, Recurring [Member] | Commercial mortgage loans |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Purchases and issuances | 500,000 | 40,600,000 |
Transfers Into Level 3 | 0 | 0 |
Transfers out of Level 3 | 0 | 0 |
Asset reclassification | 0 | 0 |
Settlements and sales | (184,400,000) | (61,500,000) |
Increase (decrease) in fair value included in net income | (11,300,000) | (16,300,000) |
Fair Market Value |  | 274,900,000 | 312,100,000
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 | 400,000 | 18,300,000 |
Transfers Into Level 3 | 0 | 0 |
Transfers out of Level 3 | 0 | 0 |
Asset reclassification | 10,900,000 | 0 |
Settlements and sales | (35,500,000) | (41,900,000) |
Increase (decrease) in fair value included in net income | 400,000 | 1,500,000 |
Fair Market Value |  | 84,800,000 | 106,900,000
Assets | Level 3 | Fair Value, Recurring [Member] | Asset-backed |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Purchases and issuances | 56,100,000 | 21,900,000 |
Transfers Into Level 3 | 0 | 5,300,000 |
Transfers out of Level 3 | 0 | (5,500,000) |
Asset reclassification | 0 | 0 |
Settlements and sales | (12,400,000) | (7,600,000) |
Increase (decrease) in fair value included in net income | 1,000,000.0 | 2,000,000.0 |
Fair Market Value |  | 82,900,000 | 66,800,000
Assets | Level 3 | Fair Value, Recurring [Member] | Global corporate securities |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Purchases and issuances | 4,400,000 | 0 |
Transfers Into Level 3 | 0 | 0 |
Transfers out of Level 3 | 0 | 0 |
Asset reclassification | 0 | 0 |
Settlements and sales | (300,000) | (400,000) |
Increase (decrease) in fair value included in net income | 0 | (200,000) |
Fair Market Value |  | 14,400,000 | 15,000,000.0
Assets | Level 3 | Fair Value, Recurring [Member] | Short-term investments |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Purchases and issuances | 0 | 18,200,000 |
Transfers Into Level 3 | 0 | 0 |
Transfers out of Level 3 | 0 | 0 |
Asset reclassification | (10,900,000) | 0 |
Settlements and sales | (7,100,000) | (25,800,000) |
Increase (decrease) in fair value included in net income | 0 | 0 |
Fair Market Value |  | 18,000,000.0 | 25,600,000
Assets | Level 3 | Fair Value, Recurring [Member] | Equity Securities |  |  |
Fair Value, Assets Measured on Recurring Basis, Calculation [Roll Forward] |  |  |
Purchases and issuances |  | 0 |
Transfers Into Level 3 |  | 0 |
Transfers out of Level 3 |  | (6,600,000) |
Asset reclassification |  | 0 |
Settlements and sales |  | 0 |
Increase (decrease) in fair value included in net income |  | 0 |
Fair Market Value |  |  | 6,600,000
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 |  | 14,900,000 | $ 0
Purchases and issuances | 10,000,000.0 | 14,700,000 |
Transfers Into Level 3 | 0 | 0 |
Transfers out of Level 3 | 0 | 0 |
Asset reclassification | 0 | 0 |
Settlements and sales | 0 | 0 |
Increase (decrease) in fair value included in net income | $ (700,000) | $ 200,000 |

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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 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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