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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 ($) | Jan. 31, 2015 USD ($) | Dec. 31, 2020 USD ($) Investment | Dec. 31, 2019 USD ($) Investment | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2020 GBP (£) Investment | Dec. 31, 2020 CAD ($) Investment | Dec. 31, 2020 AUD ($) Investment | Dec. 31, 2020 CHF (SFr) Investment | Dec. 31, 2020 SGD ($) Investment | Dec. 31, 2019 GBP (£) Investment | Dec. 31, 2019 CAD ($) Investment | Dec. 31, 2019 AUD ($) Investment | Dec. 31, 2019 CHF (SFr) Investment | Dec. 31, 2019 SGD ($) Investment | Dec. 27, 2019 USD ($) | Dec. 23, 2019 USD ($) | Sep. 19, 2018 USD ($) | May 01, 2018 USD ($)
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other investments (equity method) |  |  | $ 109.4 | $ 111.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total as percent of investable assets |  |  | 58.60% | 54.40% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investable assets held by the Company |  |  | $ 7,500.0 | $ 7,800.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investable assets held by insurance regulators |  |  | $ 541.4 | 513.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum capital required | £ |  |  |  |  |  |  | £ 0.4 |  |  |  |  | £ 0.4 |  |  |  |  |  |  |  |
Percentage of reinsurance liabilities |  |  | 100.00% |  |  |  | 100.00% | 100.00% | 100.00% | 100.00% | 100.00% |  |  |  |  |  |  |  |  |
Reinsurance liabilities |  |  | $ 1,455.3 | 1,374.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held-in-trust |  |  | 1,478.0 | 1,395.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory deposits |  |  | 6.7 | 6.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deposit with states |  |  | $ 6.1 | $ 6.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of investments | Investment |  |  | 1 | 1 |  |  | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |  |  |  |
Investment in the period |  |  | $ 0.5 | $ 1.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
MVI |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment in the period |  | $ 0.8 | 0.0 | 0.0 | $ 0.2 | $ 0.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |
MVI | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment in the period | $ 0.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Multi-Line Insurer |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other investments (equity method) |  |  | 0.3 |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.2 | $ 5.0 |  |
Investment in the period |  |  | 0.5 | 0.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Us Multi Beneficiary Trust Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum trust fund |  |  | 20.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held-in-trust |  |  | 572.3 | 846.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Us Surplus Lines Trust Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held-in-trust |  |  | $ 208.0 | 215.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Canadian Trust Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held-in-trust |  |  |  |  |  |  |  | $ 156.8 |  |  |  |  | $ 155.4 |  |  |  |  |  |  |
Australian Trust Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held-in-trust |  |  |  |  |  |  |  |  | $ 256.6 |  |  |  |  | $ 226.9 |  |  |  |  |  |
Swiss Trust Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held-in-trust | SFr |  |  |  |  |  |  |  |  |  | SFr 8.0 |  |  |  |  | SFr 8.4 |  |  |  |  |
Singapore Trust Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held-in-trust |  |  |  |  |  |  |  |  |  |  | $ 148.9 |  |  |  |  | $ 148.9 |  |  |  |
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 |  |  | $ 382.8 | 427.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Limited Partner | Real estate fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted assets |  |  |  |  | $ 100.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other investments (equity method) |  |  | $ 109.4 | $ 111.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 13.8 | $ 86.2

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