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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 | 9 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |  |  |
Beginning balance, net derivative assets (liabilities) | $ (13.1) | $ 2.0 | $ 13.5 | $ 5.5
Total realized/unrealized gains (losses) included in net income, net derivative assets (liabilities) | $ 18.3 | $ 4.5 | $ (9.5) | $ (0.2)
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 | 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
Net purchases, sales, issuances and settlements, net derivative assets (liabilities) | $ (0.3) | $ 2.8 | $ 0.9 | $ 4.0
Transfers into Level 3, net derivative assets (liabilities) |  | (0.1) |  | (0.1)
Ending balance, net derivative assets (liabilities) | 4.9 | 9.2 | 4.9 | 9.2
Changes in unrealized gains (losses) included in net income relating to positions still held, net derivative assets (liabilities) | 20.1 | 4.3 | (10.6) | 0.7
Gross purchases, sales, issuances and settlements |  |  |  |
Purchases, net derivative assets (liabilities) | (2.2) | 2.9 | (1.3) | 3.2
Sales, net derivative assets (liabilities) | 1.9 | (0.1) | 2.2 | 0.8
Net purchases, sales, issuances and settlements, net derivative assets (liabilities) | (0.3) | 2.8 | 0.9 | 4.0
Investment and universal life contracts |  |  |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |  |  |
Beginning balance, liabilities | (863.7) | (230.0) | (578.4) | (115.5)
Total realized/unrealized gains (losses) included in net income, liabilities | $ (222.2) | $ (23.8) | $ (221.2) | $ (84.4)
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 | 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
Net purchases, sales, issuances and settlements, liabilities | $ (142.9) | $ (114.2) | $ (429.2) | $ (168.1)
Ending balance, liabilities | (1,228.8) | (368.0) | (1,228.8) | (368.0)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities | (214.3) | (64.5) | (219.7) | (79.2)
Gross purchases, sales, issuances and settlements |  |  |  |
Issuances, liabilities | (155.1) | (130.9) | (469.1) | (221.0)
Settlements, liabilities | 12.2 | 16.7 | 39.9 | 52.9
Net purchases, sales, issuances and settlements, liabilities | (142.9) | (114.2) | (429.2) | (168.1)
Funds withheld payable embedded derivative |  |  |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |  |  |
Beginning balance, liabilities | 2,825.2 | 2,997.0 | 3,014.5 | 2,567.1
Total realized/unrealized gains (losses) included in net income, liabilities | $ (266.0) | $ (776.8) | $ (455.3) | $ (346.9)
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 | 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
Ending balance, liabilities | $ 2,559.2 | $ 2,220.2 | $ 2,559.2 | $ 2,220.2
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities | (266.0) | (776.8) | (455.3) | (346.9)
Fixed maturities | Available-for-sale |  |  |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |  |  |
Beginning balance, assets | 4,327.8 | 3,771.0 | 3,898.2 | 3,636.8
Total realized/unrealized gains (losses) included in net income, assets | $ (21.5) | $ 8.6 | $ (5.5) | $ 1.8
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 | 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
Total realized/unrealized gains (losses) included in other comprehensive income, assets | $ 7.2 | $ 55.9 | $ 11.6 | $ 10.2
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 | 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, assets | $ (2.0) | $ (76.1) | $ 310.5 | $ 12.2
Transfers into Level 3, assets |  | 4.6 | 121.5 | 214.6
Transfers out of Level 3, assets | (94.2) | (17.9) | (119.0) | (129.5)
Ending balance, assets | 4,217.3 | 3,746.1 | 4,217.3 | 3,746.1
Changes in unrealized gains (losses) included in net income relating to positions still held, assets | (14.6) | 8.3 | 6.3 | 1.1
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets | 6.8 | 57.7 | 11.4 | 8.3
Gross purchases, sales, issuances and settlements |  |  |  |
Purchases, assets | 218.8 | 219.3 | 1,056.6 | 790.7
Sales, assets | (28.9) | (85.2) | (362.2) | (383.7)
Settlements, assets | (191.9) | (210.2) | (383.9) | (394.8)
Net purchases, sales, issuances and settlements, assets | (2.0) | (76.1) | 310.5 | 12.2
States and political subdivisions | Available-for-sale |  |  |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |  |  |
Beginning balance, assets | 67.8 | 85.0 | 66.8 | 69.9
Total realized/unrealized gains (losses) included in other comprehensive income, assets | $ 0.9 | $ 3.6 | $ 2.8 | $ 1.6
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 | 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, assets | $ (0.5) | $ (0.4) | $ (1.4) | $ 1.2
Transfers into Level 3, assets |  |  |  | 15.5
Transfers out of Level 3, assets |  | (17.9) |  | (17.9)
Ending balance, assets | 68.2 | 70.3 | 68.2 | 70.3
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets | 0.9 | 3.6 | 2.8 | 1.7
Gross purchases, sales, issuances and settlements |  |  |  |
Purchases, assets |  |  |  | 2.5
Settlements, assets | (0.5) | (0.4) | (1.4) | (1.3)
Net purchases, sales, issuances and settlements, assets | (0.5) | (0.4) | (1.4) | 1.2
Corporate debt securities | Available-for-sale |  |  |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |  |  |
Beginning balance, assets | 2,444.0 | 2,228.6 | 2,350.4 | 2,305.9
Total realized/unrealized gains (losses) included in net income, assets | $ (21.5) | $ 8.5 | $ (5.6) | $ 1.7
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 | 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
Total realized/unrealized gains (losses) included in other comprehensive income, assets | $ 3.8 | $ 35.0 | $ (4.1) | $ 6.1
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 | 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, assets | $ (10.9) | $ 34.3 | $ 74.7 | $ (66.2)
Transfers into Level 3, assets |  |  |  | 58.9
Ending balance, assets | 2,415.4 | 2,306.4 | 2,415.4 | 2,306.4
Changes in unrealized gains (losses) included in net income relating to positions still held, assets | (14.6) | 8.2 | 6.3 | 1.0
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets | 3.5 | 36.8 | (5.0) | 5.7
Gross purchases, sales, issuances and settlements |  |  |  |
Purchases, assets | 125.7 | 169.1 | 449.9 | 446.5
Sales, assets | (28.9) | (57.0) | (184.5) | (296.4)
Settlements, assets | (107.7) | (77.8) | (190.7) | (216.3)
Net purchases, sales, issuances and settlements, assets | (10.9) | 34.3 | 74.7 | (66.2)
Commercial mortgage-backed securities | Available-for-sale |  |  |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |  |  |
Beginning balance, assets | 2.6 | 2.8 | 2.7 | 3.0
Total realized/unrealized gains (losses) included in other comprehensive income, assets |  | 0.1 | $ 0.1 | 0.1
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 |
Net purchases, sales, issuances and settlements, assets | (0.1) | (0.1) | $ (0.3) | (0.3)
Ending balance, assets | 2.5 | 2.8 | 2.5 | 2.8
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets |  | 0.1 | 0.1 | 0.1
Gross purchases, sales, issuances and settlements |  |  |  |
Settlements, assets | (0.1) | (0.1) | (0.3) | (0.3)
Net purchases, sales, issuances and settlements, assets | (0.1) | (0.1) | (0.3) | (0.3)
Collateralized debt obligations | Available-for-sale |  |  |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |  |  |
Beginning balance, assets |  | 104.7 |  | 75.4
Total realized/unrealized gains (losses) included in other comprehensive income, assets |  | $ 3.0 |  | $ 0.2
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
Net purchases, sales, issuances and settlements, assets |  | $ (13.8) |  | $ 18.3
Ending balance, assets |  | 93.9 |  | 93.9
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets |  | 2.9 |  | 1.0
Gross purchases, sales, issuances and settlements |  |  |  |
Purchases, assets |  | 3.0 |  | 35.1
Settlements, assets |  | (16.8) |  | (16.8)
Net purchases, sales, issuances and settlements, assets |  | (13.8) |  | 18.3
Other debt obligations | Available-for-sale |  |  |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |  |  |
Beginning balance, assets | 1,813.4 | 1,349.9 | 1,478.3 | 1,182.6
Total realized/unrealized gains (losses) included in net income, assets |  | 0.1 | $ 0.1 | 0.1
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 |
Total realized/unrealized gains (losses) included in other comprehensive income, assets | $ 2.5 | $ 14.2 | $ 12.8 | $ 2.2
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 | 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, assets | $ 9.5 | $ (96.1) | $ 237.5 | $ 59.2
Transfers into Level 3, assets |  | 4.6 | 121.5 | 140.2
Transfers out of Level 3, assets | (94.2) |  | (119.0) | (111.6)
Ending balance, assets | 1,731.2 | 1,272.7 | 1,731.2 | 1,272.7
Changes in unrealized gains (losses) included in net income relating to positions still held, assets |  | 0.1 |  | 0.1
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets | 2.4 | 14.3 | 13.5 | (0.2)
Gross purchases, sales, issuances and settlements |  |  |  |
Purchases, assets | 93.1 | 47.2 | 606.7 | 306.6
Sales, assets |  | (28.2) | (177.7) | (87.3)
Settlements, assets | (83.6) | (115.1) | (191.5) | (160.1)
Net purchases, sales, issuances and settlements, assets | 9.5 | (96.1) | 237.5 | 59.2
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 | 119.3 | 173.5 | 131.2 | 165.1
Total realized/unrealized gains (losses) included in net income, assets | $ (2.5) | $ (5.3) | $ (2.7) | $ (16.7)
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 | 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
Net purchases, sales, issuances and settlements, assets | $ 19.7 | $ (26.7) | $ 8.0 | $ (6.9)
Ending balance, assets | 136.5 | 141.5 | 136.5 | 141.5
Changes in unrealized gains (losses) included in net income relating to positions still held, assets | 0.4 | (2.0) | 4.7 | (8.6)
Gross purchases, sales, issuances and settlements |  |  |  |
Purchases, assets | 58.7 | 25.5 | 119.6 | 144.2
Sales, assets |  |  | (1.5) |
Settlements, assets | (39.0) | (52.2) | (110.1) | (151.1)
Net purchases, sales, issuances and settlements, assets | 19.7 | (26.7) | 8.0 | (6.9)
Fixed maturities, trading |  |  |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |  |  |
Beginning balance, assets | 491.9 | 445.2 | 562.6 | 415.8
Total realized/unrealized gains (losses) included in net income, assets | $ (3.4) | $ 7.5 | $ (15.3) | $ 5.8
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 | 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
Net purchases, sales, issuances and settlements, assets | $ 11.9 | $ 40.1 | $ (42.1) | $ 71.2
Transfers out of Level 3, assets |  |  | (4.8) |
Ending balance, assets | 500.4 | 492.8 | 500.4 | 492.8
Changes in unrealized gains (losses) included in net income relating to positions still held, assets | (14.8) | 5.2 | (14.9) | 5.8
Gross purchases, sales, issuances and settlements |  |  |  |
Purchases, assets | 68.5 | 75.4 | 231.1 | 296.5
Sales, assets | (22.4) | (27.5) | (144.0) | (148.0)
Settlements, assets | (34.2) | (7.8) | (129.2) | (77.3)
Net purchases, sales, issuances and settlements, assets | 11.9 | 40.1 | (42.1) | 71.2
Separate account assets |  |  |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |  |  |
Beginning balance, assets | 741.4 | 718.4 | 726.7 | 752.8
Total realized/unrealized gains (losses) included in net income, assets | $ 2.0 | $ 2.2 | $ 16.7 | $ (30.4)
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 | 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
Net purchases, sales, issuances and settlements, assets |  | $ (6.8) |  | $ (8.6)
Ending balance, assets | $ 743.4 | 713.8 | $ 743.4 | 713.8
Changes in unrealized gains (losses) included in net income relating to positions still held, assets | 32.4 | (13.5) | 24.6 | (20.6)
Gross purchases, sales, issuances and settlements |  |  |  |
Sales, assets | (42.6) |  | (42.6) | (30.3)
Issuances, assets |  | (49.5) |  | (60.0)
Settlements, assets | $ 42.6 | 42.7 | $ 42.6 | 81.7
Net purchases, sales, issuances and settlements, assets |  | $ (6.8) |  | $ (8.6)

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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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