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Third-Party Reinsurance - Effects of Reinsurance (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Earned premiums: |  |  |
Net earned premiums | $ 1,440.9 | $ 1,076.7 | $ 664.2
Losses and LAE: |  |  |
Net incurred losses and LAE | 726.8 | 536.4 | 314.8
Ark Insurance Holdings Limited/Outrigger |  |  |
Written premiums: |  |  |
Direct | 931.9 | 760.4 | 619.5
Assumed | 966.5 | 691.6 | 439.2
Gross written premiums | 1,898.4 | 1,452.0 | 1,058.7
Ceded | (377.5) | (256.8) | (199.6)
Net written premiums | 1,520.9 | 1,195.2 | 859.1
Earned premiums: |  |  |
Direct | 848.1 | 655.5 | 556.0
Assumed | 925.6 | 668.7 | 330.4
Gross earned premiums | 1,773.7 | 1,324.2 | 886.4
Ceded | (364.0) | (280.8) | (249.1)
Net earned premiums | 1,409.7 | 1,043.4 | 637.3
Losses and LAE: |  |  |
Gross | 782.5 | 814.9 | 442.9
Ceded | (55.7) | (278.5) | (128.1)
Net incurred losses and LAE | 726.8 | $ 536.4 | $ 314.8
Ark Insurance Holdings Limited/Outrigger | Ark |  |  |
Written premiums: |  |  |
Ceded | 110.0 |  |
Earned premiums: |  |  |
Ceded | 104.3 |  |
Losses and LAE: |  |  |
Ceded | $ 15.6 |  |

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