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Commitments and Contingencies - Additional Information (Details) £ in Millions, SFr in Millions, $ in Millions, $ in Millions, $ in Millions, $ in Millions | 1 Months Ended | 12 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 31, 2021 USD ($) | Dec. 31, 2023 USD ($) Investment | Dec. 31, 2022 USD ($) Investment | Dec. 31, 2021 USD ($) | Dec. 31, 2023 GBP (£) Investment | Dec. 31, 2023 CAD ($) Investment | Dec. 31, 2023 AUD ($) Investment | Dec. 31, 2023 CHF (SFr) Investment | Dec. 31, 2023 SGD ($) Investment | Dec. 31, 2022 GBP (£) Investment | Dec. 31, 2022 CAD ($) Investment | Dec. 31, 2022 AUD ($) Investment | Dec. 31, 2022 CHF (SFr) Investment | Dec. 31, 2022 SGD ($) Investment | Apr. 01, 2022 USD ($) | Sep. 30, 2021 USD ($) | Dec. 23, 2019 USD ($) | Dec. 20, 2017 USD ($)
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other investments (equity method) |  | $ 209.3 | $ 221.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total as percent of investable assets |  | 50.20% | 59.40% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment in the period |  | $ 0.4 | $ 2.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment to invest in private assets |  | 99.0 | 377.9 | $ 205.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investable assets held by the Company |  | 7,500.0 | 7,100.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investable assets held by insurance regulators |  | $ 517.4 | 489.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum capital required | £ |  |  |  |  | £ 0.5 |  |  |  |  | £ 0.4 |  |  |  |  |  |  |  |
Percentage of reinsurance liabilities |  | 100.00% |  |  | 100.00% | 100.00% | 100.00% | 100.00% | 100.00% |  |  |  |  |  |  |  |  |
Reinsurance liabilities |  | $ 823.5 | 1,166.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held-in-trust |  | 1,016.9 | 1,389.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory deposits |  | 6.8 | 6.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deposit with states |  | $ 6.4 | $ 6.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of investments | Investment |  | 1 | 1 |  | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |  |  |  |
MVI |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment in the period | $ 0.8 | $ 0.0 | $ 0.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Multi-Line Insurer |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other investments (equity method) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 5.0 |
Investment in the period |  | 0.4 | 1.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Us Multi Beneficiary Trust Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum trust fund |  | 20.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held-in-trust |  | 394.7 | 509.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Us Surplus Lines Trust Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held-in-trust |  | 126.6 | 215.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Canadian Trust Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held-in-trust |  | 168.4 |  |  |  | $ 228.4 |  |  |  |  | $ 185.8 |  |  |  |  |  |  |
Australian Trust Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held-in-trust |  | 86.9 |  |  |  |  | $ 131.0 |  |  |  |  | $ 183.9 |  |  |  |  |  |
Swiss Trust Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held-in-trust |  | 11.4 |  |  |  |  |  | SFr 9.9 |  |  |  |  | SFr 9.4 |  |  |  |  |
Singapore Trust Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held-in-trust |  | $ 144.0 |  |  |  |  |  |  | $ 192.1 |  |  |  |  | $ 174.7 |  |  |  |
Bermuda |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of reinsurance liabilities |  | 100.00% |  |  | 100.00% | 100.00% | 100.00% | 100.00% | 100.00% |  |  |  |  |  |  |  |  |
Reinsurance liabilities |  | $ 320.6 | $ 380.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Limited Partner |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 10.0 | $ 20.0 |  | $ 100.0

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