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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 | 6 Months Ended
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Jun. 30, 2025 | Jun. 30, 2024 | Jun. 30, 2025 | Jun. 30, 2024
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |  |  |
Beginning balance, net derivative assets (liabilities) | $ (1.8) | $ 0.9 | $ 13.5 | $ 5.5
Total realized/unrealized gains (losses) included in net income, net derivative assets (liabilities) | $ (11.1) | $ 0.3 | $ (27.8) | $ (4.7)
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) | $ (0.1) | $ 0.8 | $ 1.2 | $ 1.2
Transfers out of Level 3, net derivative assets (liabilities) | (0.1) |  |  |
Ending balance, net derivative assets (liabilities) | (13.1) | 2.0 | (13.1) | 2.0
Changes in unrealized gains (losses) included in net income relating to positions still held, net derivative assets (liabilities) | (12.6) | 0.9 | (30.6) | (3.6)
Gross purchases, sales, issuances and settlements |  |  |  |
Purchases, net derivative assets (liabilities) | 0.2 | 0.1 | 0.9 | 0.3
Sales, net derivative assets (liabilities) | (0.3) | 0.7 | 0.3 | 0.9
Net purchases, sales, issuances and settlements, net derivative assets (liabilities) | (0.1) | 0.8 | 1.2 | 1.2
Investment and universal life contracts |  |  |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |  |  |
Beginning balance, liabilities | (522.7) | (174.7) | (578.4) | (115.5)
Total realized/unrealized gains (losses) included in net income, liabilities | $ (192.4) | $ (17.2) | $ 1.0 | $ (60.6)
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 | $ (148.6) | $ (38.1) | $ (286.3) | $ (53.9)
Ending balance, liabilities | (863.7) | (230.0) | (863.7) | (230.0)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities | (7.8) | (49.5) | 0.3 | (59.5)
Gross purchases, sales, issuances and settlements |  |  |  |
Issuances, liabilities | (160.1) | (56.3) | (314.0) | (90.1)
Settlements, liabilities | 11.5 | 18.2 | 27.7 | 36.2
Net purchases, sales, issuances and settlements, liabilities | (148.6) | (38.1) | (286.3) | (53.9)
Funds withheld payable embedded derivative |  |  |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |  |  |
Beginning balance, liabilities | 2,804.8 | 2,764.1 | 3,014.5 | 2,567.1
Total realized/unrealized gains (losses) included in net income, liabilities | $ 20.4 | $ 232.9 | $ (189.3) | $ 429.9
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 | $ 2,825.2 | $ 2,997.0 | $ 2,825.2 | $ 2,997.0
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities | 20.4 | 232.9 | (189.4) | 429.9
Fixed maturities | Available-for-sale |  |  |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |  |  |
Beginning balance, assets | 3,950.2 | 3,539.5 | 3,898.2 | 3,636.8
Total realized/unrealized gains (losses) included in net income, assets | $ 13.0 | $ (1.6) | $ 16.0 | $ (6.8)
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 | $ 4.3 | $ (24.2) | $ 4.4 | $ (45.7)
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 | $ 238.8 | $ 166.8 | $ 312.5 | $ 88.3
Transfers into Level 3, assets | 121.5 | 90.5 | 121.5 | 210.0
Transfers out of Level 3, assets |  |  | (24.8) | (111.6)
Ending balance, assets | 4,327.8 | 3,771.0 | 4,327.8 | 3,771.0
Changes in unrealized gains (losses) included in net income relating to positions still held, assets | 13.0 | (1.6) | 27.4 | (6.6)
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets | 4.6 | (23.2) | 3.9 | (48.7)
Gross purchases, sales, issuances and settlements |  |  |  |
Purchases, assets | 377.9 | 304.9 | 837.8 | 571.4
Sales, assets | (25.6) | (18.3) | (333.3) | (298.5)
Settlements, assets | (113.5) | (119.8) | (192.0) | (184.6)
Net purchases, sales, issuances and settlements, assets | 238.8 | 166.8 | 312.5 | 88.3
States and political subdivisions | Available-for-sale |  |  |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |  |  |
Beginning balance, assets | 68.6 | 68.2 | 66.8 | 69.9
Total realized/unrealized gains (losses) included in other comprehensive income, assets | $ (0.4) | $ (0.7) | $ 1.9 | $ (2.0)
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 | $ (0.4) | $ 2.0 | $ (0.9) | $ 1.6
Transfers into Level 3, assets |  | 15.5 |  | 15.5
Ending balance, assets | 67.8 | 85.0 | 67.8 | 85.0
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets | (0.4) | (0.7) | 1.9 | (2.0)
Gross purchases, sales, issuances and settlements |  |  |  |
Purchases, assets |  | 2.5 |  | 2.5
Settlements, assets | (0.4) | (0.5) | (0.9) | (0.9)
Net purchases, sales, issuances and settlements, assets | (0.4) | 2.0 | (0.9) | 1.6
Corporate debt securities | Available-for-sale |  |  |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |  |  |
Beginning balance, assets | 2,419.3 | 2,202.6 | 2,350.4 | 2,305.9
Total realized/unrealized gains (losses) included in net income, assets | $ 13.0 | $ (1.6) | $ 15.9 | $ (6.8)
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 | $ (2.7) | $ (17.7) | $ (7.9) | $ (28.9)
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 | $ 14.4 | $ 25.9 | $ 85.6 | $ (100.5)
Transfers into Level 3, assets |  | 19.4 |  | 58.9
Ending balance, assets | 2,444.0 | 2,228.6 | 2,444.0 | 2,228.6
Changes in unrealized gains (losses) included in net income relating to positions still held, assets | 13.0 | (1.6) | 27.4 | (6.6)
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets | (2.4) | (17.5) | (8.7) | (30.3)
Gross purchases, sales, issuances and settlements |  |  |  |
Purchases, assets | 89.0 | 141.9 | 324.2 | 277.4
Sales, assets | (25.6) | (18.3) | (155.6) | (239.4)
Settlements, assets | (49.0) | (97.7) | (83.0) | (138.5)
Net purchases, sales, issuances and settlements, assets | 14.4 | 25.9 | 85.6 | (100.5)
Commercial mortgage-backed securities | Available-for-sale |  |  |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |  |  |
Beginning balance, assets | 2.6 | 2.9 | 2.7 | 3.0
Total realized/unrealized gains (losses) included in other comprehensive income, assets |  |  | $ 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) | $ (0.2) | (0.2)
Ending balance, assets | 2.6 | 2.8 | 2.6 | 2.8
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets |  |  | 0.1 |
Gross purchases, sales, issuances and settlements |  |  |  |
Settlements, assets |  | (0.1) | (0.2) | (0.2)
Net purchases, sales, issuances and settlements, assets |  | (0.1) | (0.2) | (0.2)
Collateralized debt obligations | Available-for-sale |  |  |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |  |  |
Beginning balance, assets |  | 99.4 |  | 75.4
Total realized/unrealized gains (losses) included in other comprehensive income, assets |  | $ (1.1) |  | $ (2.8)
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 |  | $ 6.4 |  | $ 32.1
Ending balance, assets |  | 104.7 |  | 104.7
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets |  | (0.3) |  | (3.7)
Gross purchases, sales, issuances and settlements |  |  |  |
Purchases, assets |  | 6.4 |  | 32.1
Net purchases, sales, issuances and settlements, assets |  | 6.4 |  | 32.1
Other debt obligations | Available-for-sale |  |  |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |  |  |
Beginning balance, assets | 1,459.7 | 1,166.4 | 1,478.3 | 1,182.6
Total realized/unrealized gains (losses) included in net income, assets |  |  | $ 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 | $ 7.4 | $ (4.7) | $ 10.3 | $ (12.0)
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 | $ 224.8 | $ 132.6 | $ 228.0 | $ 155.3
Transfers into Level 3, assets | 121.5 | 55.6 | 121.5 | 135.6
Transfers out of Level 3, assets |  |  | (24.8) | (111.6)
Ending balance, assets | 1,813.4 | 1,349.9 | 1,813.4 | 1,349.9
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets | 7.4 | (4.7) | 10.6 | (12.7)
Gross purchases, sales, issuances and settlements |  |  |  |
Purchases, assets | 288.9 | 154.1 | 513.6 | 259.4
Sales, assets |  |  | (177.7) | (59.1)
Settlements, assets | (64.1) | (21.5) | (107.9) | (45.0)
Net purchases, sales, issuances and settlements, assets | 224.8 | 132.6 | 228.0 | 155.3
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 | 118.6 | 165.8 | 131.2 | 165.1
Total realized/unrealized gains (losses) included in net income, assets | $ (2.9) | $ (6.1) | $ (0.2) | $ (11.4)
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 | $ 3.6 | $ 13.8 | $ (11.7) | $ 19.8
Ending balance, assets | 119.3 | 173.5 | 119.3 | 173.5
Changes in unrealized gains (losses) included in net income relating to positions still held, assets | 4.2 | (3.2) | 4.2 | (6.6)
Gross purchases, sales, issuances and settlements |  |  |  |
Purchases, assets | 39.6 | 66.8 | 60.9 | 118.7
Sales, assets |  |  | (1.5) |
Settlements, assets | (36.0) | (53.0) | (71.1) | (98.9)
Net purchases, sales, issuances and settlements, assets | 3.6 | 13.8 | (11.7) | 19.8
Fixed maturities, trading |  |  |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |  |  |
Beginning balance, assets | 571.1 | 470.2 | 562.6 | 415.8
Total realized/unrealized gains (losses) included in net income, assets | $ (11.2) | $ 1.8 | $ (11.9) | $ (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
Net purchases, sales, issuances and settlements, assets | $ (68.0) | $ (26.8) | $ (54.0) | $ 31.1
Transfers out of Level 3, assets |  |  | (4.8) |
Ending balance, assets | 491.9 | 445.2 | 491.9 | 445.2
Changes in unrealized gains (losses) included in net income relating to positions still held, assets | (11.0) | 2.2 | (11.2) | 2.1
Gross purchases, sales, issuances and settlements |  |  |  |
Purchases, assets | 98.3 | 33.8 | 162.6 | 221.1
Sales, assets | (76.8) | (23.5) | (121.6) | (120.5)
Settlements, assets | (89.5) | (37.1) | (95.0) | (69.5)
Net purchases, sales, issuances and settlements, assets | (68.0) | (26.8) | (54.0) | 31.1
Separate account assets |  |  |  |
Changes in Level 3 fair value measurements rollforward, assets and liabilities |  |  |  |
Beginning balance, assets | 738.2 | 725.6 | 726.7 | 752.8
Total realized/unrealized gains (losses) included in net income, assets | $ 3.2 | $ (6.3) | $ 14.7 | $ (32.6)
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 |  | $ (0.9) |  | $ (1.8)
Ending balance, assets | $ 741.4 | 718.4 | $ 741.4 | 718.4
Changes in unrealized gains (losses) included in net income relating to positions still held, assets | $ (1.2) | (31.5) | $ (7.8) | (19.1)
Gross purchases, sales, issuances and settlements |  |  |  |
Sales, assets |  |  |  | (30.3)
Issuances, assets |  | (0.9) |  | (10.5)
Settlements, assets |  |  |  | 39.0
Net purchases, sales, issuances and settlements, assets |  | $ (0.9) |  | $ (1.8)

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