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Fair Value Measurements - Changes in Level 3 Fair Value Measurements (Details) - Recurring Fair Value Measurements - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2025 | Mar. 31, 2024
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] | Gain (Loss) on Investments, Change In Fair Value Of Funds Withheld Embedded Derivative | Gain (Loss) on Investments, Change In Fair Value Of Funds Withheld Embedded Derivative
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Asset, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration] | Other Comprehensive Income Available for Sale Securities Period Increase Decrease Net of Tax, Other Comprehensive Income Derivatives Period Increase (Decrease), Net of Tax | Other Comprehensive Income Available for Sale Securities Period Increase Decrease Net of Tax, Other Comprehensive Income Derivatives Period Increase (Decrease), Net of Tax
Fair Value, Liability, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] | Gain (Loss) on Investments, Change In Fair Value Of Funds Withheld Embedded Derivative | Gain (Loss) on Investments, Change In Fair Value Of Funds Withheld Embedded Derivative
Fair Value, Liability, Recurring Basis, Unobservable Input Reconciliation, Liability, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration] | Other Comprehensive Income Available for Sale Securities Period Increase Decrease Net of Tax, Other Comprehensive Income Derivatives Period Increase (Decrease), Net of Tax | Other Comprehensive Income Available for Sale Securities Period Increase Decrease Net of Tax, Other Comprehensive Income Derivatives Period Increase (Decrease), Net of Tax
Beginning balance, net derivative assets (liabilities) | $ 13.5 | $ 5.5
Total realized/unrealized gains (losses) included in net income, net derivative assets (liabilities) | $ (16.7) | $ (5.0)
Fair Value, Net Derivative Asset (Liability), Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration] | Gain (Loss) on Investments, Change In Fair Value Of Funds Withheld Embedded Derivative | Gain (Loss) on Investments, Change In Fair Value Of Funds Withheld Embedded Derivative
Fair Value, Net Derivative Asset (Liability), Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration] | Other Comprehensive Income Available for Sale Securities Period Increase Decrease Net of Tax, Other Comprehensive Income Derivatives Period Increase (Decrease), Net of Tax | Other Comprehensive Income Available for Sale Securities Period Increase Decrease Net of Tax, Other Comprehensive Income Derivatives Period Increase (Decrease), Net of Tax
Net purchases, sales, issuances and settlements, net derivative assets (liabilities) | $ 1.3 | $ 0.4
Transfers out of Level 3, net derivative assets (liabilities) | 0.1 |
Ending balance, net derivative assets (liabilities) | (1.8) | 0.9
Changes in unrealized gains (losses) included in net income relating to positions still held, net derivative assets (liabilities) | (18.5) | (4.5)
Gross purchases, sales, issuances and settlements |  |
Purchases, net derivative assets (liabilities) | 0.7 | 0.2
Sales, net derivative assets (liabilities) | 0.6 | 0.2
Net purchases, sales, issuances and settlements, net derivative assets (liabilities) | 1.3 | 0.4
Investment and universal life contracts |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |
Beginning balance, liabilities | (578.4) | (115.5)
Total realized/unrealized gains (losses) included in net income, liabilities | 193.4 | (43.4)
Net purchases, sales, issuances and settlements, liabilities | (137.7) | (15.8)
Ending balance, liabilities | (522.7) | (174.7)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities | 193.6 | (44.5)
Gross purchases, sales, issuances and settlements |  |
Issuances, liabilities | (153.9) | (33.8)
Settlements, liabilities | 16.2 | 18.0
Net purchases, sales, issuances and settlements, liabilities | (137.7) | (15.8)
Funds withheld payable embedded derivative |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |
Beginning balance, liabilities | 3,014.5 | 2,567.1
Total realized/unrealized gains (losses) included in net income, liabilities | (209.7) | 197.0
Ending balance, liabilities | 2,804.8 | 2,764.1
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities | (209.7) | 197.0
Fixed maturities | Available-for-sale |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |
Beginning balance, assets | 3,898.2 | 3,636.8
Total realized/unrealized gains (losses) included in net income, assets | 3.0 | (5.2)
Total realized/unrealized gains (losses) included in other comprehensive income, assets | 0.1 | (21.5)
Net purchases, sales, issuances and settlements, assets | 73.7 | (78.5)
Transfers into Level 3, assets |  | 119.5
Transfers out of Level 3, assets | (24.8) | (111.6)
Ending balance, assets | 3,950.2 | 3,539.5
Changes in unrealized gains (losses) included in net income relating to positions still held, assets | 6.4 | (3.6)
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets | 0.3 | (22.9)
Gross purchases, sales, issuances and settlements |  |
Purchases, assets | 459.9 | 266.5
Sales, assets | (307.7) | (280.2)
Settlements, assets | (78.5) | (64.8)
Net purchases, sales, issuances and settlements, assets | 73.7 | (78.5)
States and political subdivisions | Available-for-sale |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |
Beginning balance, assets | 66.8 | 69.9
Total realized/unrealized gains (losses) included in other comprehensive income, assets | 2.3 | (1.3)
Net purchases, sales, issuances and settlements, assets | (0.5) | (0.4)
Ending balance, assets | 68.6 | 68.2
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets | 2.3 | (1.3)
Gross purchases, sales, issuances and settlements |  |
Settlements, assets | (0.5) | (0.4)
Net purchases, sales, issuances and settlements, assets | (0.5) | (0.4)
Corporate debt securities | Available-for-sale |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |
Beginning balance, assets | 2,350.4 | 2,305.9
Total realized/unrealized gains (losses) included in net income, assets | 2.9 | (5.2)
Total realized/unrealized gains (losses) included in other comprehensive income, assets | (5.2) | (11.2)
Net purchases, sales, issuances and settlements, assets | 71.2 | (126.4)
Transfers into Level 3, assets |  | 39.5
Ending balance, assets | 2,419.3 | 2,202.6
Changes in unrealized gains (losses) included in net income relating to positions still held, assets | 6.4 | (3.6)
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets | (5.2) | (12.6)
Gross purchases, sales, issuances and settlements |  |
Purchases, assets | 235.2 | 135.5
Sales, assets | (130.0) | (221.1)
Settlements, assets | (34.0) | (40.8)
Net purchases, sales, issuances and settlements, assets | 71.2 | (126.4)
Commercial mortgage-backed securities | Available-for-sale |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |
Beginning balance, assets | 2.7 | 3.0
Total realized/unrealized gains (losses) included in other comprehensive income, assets | 0.1 |
Net purchases, sales, issuances and settlements, assets | (0.2) | (0.1)
Ending balance, assets | 2.6 | 2.9
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets | 0.1 |
Gross purchases, sales, issuances and settlements |  |
Settlements, assets | (0.2) | (0.1)
Net purchases, sales, issuances and settlements, assets | (0.2) | (0.1)
Collateralized debt obligations | Available-for-sale |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |
Beginning balance, assets |  | 75.4
Total realized/unrealized gains (losses) included in other comprehensive income, assets |  | (1.7)
Net purchases, sales, issuances and settlements, assets |  | 25.7
Ending balance, assets |  | 99.4
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets |  | (1.7)
Gross purchases, sales, issuances and settlements |  |
Purchases, assets |  | 25.7
Net purchases, sales, issuances and settlements, assets |  | 25.7
Other debt obligations | Available-for-sale |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |
Beginning balance, assets | 1,478.3 | 1,182.6
Total realized/unrealized gains (losses) included in net income, assets | 0.1 |
Total realized/unrealized gains (losses) included in other comprehensive income, assets | 2.9 | (7.3)
Net purchases, sales, issuances and settlements, assets | 3.2 | 22.7
Transfers into Level 3, assets |  | 80.0
Transfers out of Level 3, assets | (24.8) | (111.6)
Ending balance, assets | 1,459.7 | 1,166.4
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets | 3.1 | (7.3)
Gross purchases, sales, issuances and settlements |  |
Purchases, assets | 224.7 | 105.3
Sales, assets | (177.7) | (59.1)
Settlements, assets | (43.8) | (23.5)
Net purchases, sales, issuances and settlements, assets | 3.2 | 22.7
Mortgage loans |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |
Beginning balance, assets | 140.6 |
Net purchases, sales, issuances and settlements, assets | (140.6) |
Gross purchases, sales, issuances and settlements |  |
Settlements, assets | (140.6) |
Net purchases, sales, issuances and settlements, assets | (140.6) |
Other investments |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |
Beginning balance, assets | 131.2 | 165.1
Total realized/unrealized gains (losses) included in net income, assets | 2.7 | (5.3)
Net purchases, sales, issuances and settlements, assets | (15.3) | 6.0
Ending balance, assets | 118.6 | 165.8
Changes in unrealized gains (losses) included in net income relating to positions still held, assets | 4.6 | (3.4)
Gross purchases, sales, issuances and settlements |  |
Purchases, assets | 21.3 | 51.9
Sales, assets | (1.5) |
Settlements, assets | (35.1) | (45.9)
Net purchases, sales, issuances and settlements, assets | (15.3) | 6.0
Fixed maturities, trading |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |
Beginning balance, assets | 562.6 | 415.8
Total realized/unrealized gains (losses) included in net income, assets | (0.7) | (3.5)
Net purchases, sales, issuances and settlements, assets | 14.0 | 57.9
Transfers out of Level 3, assets | (4.8) |
Ending balance, assets | 571.1 | 470.2
Changes in unrealized gains (losses) included in net income relating to positions still held, assets | (0.2) | (3.7)
Gross purchases, sales, issuances and settlements |  |
Purchases, assets | 64.3 | 187.3
Sales, assets | (44.8) | (97.0)
Settlements, assets | (5.5) | (32.4)
Net purchases, sales, issuances and settlements, assets | 14.0 | 57.9
Separate account assets |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |
Beginning balance, assets | 726.7 | 752.8
Total realized/unrealized gains (losses) included in net income, assets | 11.5 | (26.3)
Net purchases, sales, issuances and settlements, assets |  | (0.9)
Ending balance, assets | 738.2 | 725.6
Changes in unrealized gains (losses) included in net income relating to positions still held, assets | $ (6.6) | (33.3)
Gross purchases, sales, issuances and settlements |  |
Sales, assets |  | (30.3)
Issuances, assets |  | (9.6)
Settlements, assets |  | 39.0
Net purchases, sales, issuances and settlements, assets |  | $ (0.9)

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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 Fair value 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) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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