Third-Party Reinsurance - Effects of Reinsurance (Details) - USD ($) $ in Millions |
3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|
Sep. 30, 2026 |
Sep. 30, 2025 |
Sep. 30, 2026 |
Sep. 30, 2025 |
|
| Earned premiums: | ||||
| Net earned premiums | $ 517.7 | $ 557.8 | $ 1,328.0 | |
| Loss and LAE: | ||||
| Net incurred loss and LAE | 230.4 | 258.5 | 686.8 | |
| P&C Insurance and Reinsurance (Ark/WM Outrigger) | ||||
| Written premiums: | ||||
| Direct | 302.4 | 240.0 | $ 1,033.3 | 1,013.4 |
| Assumed | 83.6 | 126.4 | 1,221.1 | 1,275.8 |
| Gross written premiums | 386.0 | 366.4 | 2,254.4 | 2,289.2 |
| Ceded | (48.9) | (79.4) | (789.8) | (695.9) |
| Net written premiums | 337.1 | 287.0 | 1,464.6 | 1,593.3 |
| Earned premiums: | ||||
| Direct | 372.4 | 334.3 | 927.3 | 840.7 |
| Assumed Premiums Earned | 446.5 | 451.8 | 925.6 | 874.9 |
| Gross earned premiums | 818.9 | 786.1 | 1,852.9 | 1,715.6 |
| Ceded | (310.1) | (243.9) | (594.1) | (451.2) |
| Net earned premiums | 508.8 | 542.2 | 1,258.8 | 1,264.4 |
| Loss and LAE: | ||||
| Gross | 340.1 | 332.0 | 958.9 | 852.2 |
| Ceded | (110.0) | (77.4) | (346.5) | (200.1) |
| Net incurred loss and LAE | 230.1 | 254.6 | 612.4 | 652.1 |
| Effects of Reinsurance Footnotes [Abstract] | ||||
| Ceded premiums written | 48.9 | 79.4 | 789.8 | 695.9 |
| Ceded premiums earned | 310.1 | 243.9 | 594.1 | 451.2 |
| Ceded | 110.0 | 77.4 | 346.5 | 200.1 |
| P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | ||||
| Written premiums: | ||||
| Ceded | (0.8) | (3.6) | (1.2) | (83.7) |
| Earned premiums: | ||||
| Ceded | (0.8) | (43.6) | (2.7) | (62.7) |
| Loss and LAE: | ||||
| Ceded | (0.0) | (2.2) | 0.2 | 24.1 |
| Effects of Reinsurance Footnotes [Abstract] | ||||
| Ceded premiums written | 0.8 | 3.6 | 1.2 | 83.7 |
| Ceded premiums earned | 0.8 | 43.6 | 2.7 | 62.7 |
| Ceded | $ 0.0 | $ 2.2 | $ (0.2) | $ (24.1) |
| X | ||||||||||
- Definition Amount of earned premiums assumed from other entities. Reference 1: http://www.xbrl.org/2003/role/exampleRef
|
| X | ||||||||||
- Definition Amount of written premiums assumed from other entities. Reference 1: http://www.xbrl.org/2003/role/exampleRef
|
| X | ||||||||||
- Definition Amount of earned premiums ceded to other entities. Reference 1: http://www.xbrl.org/2003/role/exampleRef
|
| X | ||||||||||
- Definition Amount of written premiums ceded to other entities. Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
|
| X | ||||||||||
- Definition Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums earned. Reference 1: http://www.xbrl.org/2003/role/exampleRef
|
| X | ||||||||||
- Definition Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums written. Reference 1: http://www.xbrl.org/2003/role/exampleRef
|
| X | ||||||||||
- Definition Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred and costs incurred in the claim settlement process. Reference 1: http://www.xbrl.org/2003/role/exampleRef
|
| X | ||||||||||
- Definition Amount, before effect of policies assumed or ceded, of expense related to provision for policy benefits and costs incurred. Reference 1: http://www.xbrl.org/2003/role/exampleRef
|
| X | ||||||||||
- Definition Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned. Reference 1: http://www.xbrl.org/2003/role/exampleRef
|
| X | ||||||||||
- Definition Amount of direct and assumed premiums written. Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
|
| X | ||||||||||
- Definition Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums written. Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
|
| X | ||||||||||
- References No definition available.
|
| X | ||||||||||
- Definition Amount of reduction of provision for policy benefits and costs incurred for policies ceded. Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
|
| X | ||||||||||
- References No definition available.
|
| X | ||||||||||
- Definition Effects of Reinsurance Footnotes No definition available.
|
| X | ||||||||||
- Definition Gross Premiums Earned No definition available.
|
| X | ||||||||||
- Definition Losses and Loss Adjustment Expense No definition available.
|
| X | ||||||||||
- Details
|
| X | ||||||||||
- Details
|