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v3.10.0.1

Commitments and Contingencies - Additional Information (Details) $ / shares in Units, £ in Millions, SFr in Millions, $ in Millions, $ in Millions, $ in Millions, $ in Millions | 12 Months Ended |  |  |  |  |
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Dec. 31, 2018 USD ($) Investment | Dec. 31, 2017 USD ($) Investment | Mar. 31, 2019 $ / shares | Dec. 31, 2018 GBP (£) Investment | Dec. 31, 2018 AUD ($) Investment | Dec. 31, 2018 CHF (SFr) Investment | Dec. 31, 2018 CAD ($) Investment | Dec. 31, 2018 SGD ($) Investment | Sep. 19, 2018 USD ($) | May 01, 2018 USD ($) | Dec. 31, 2017 GBP (£) Investment | Dec. 31, 2017 AUD ($) Investment | Dec. 31, 2017 CHF (SFr) Investment | Dec. 31, 2017 CAD ($) Investment | Dec. 31, 2017 SGD ($) Investment
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted assets | $ 4,419.2 | $ 4,638.8 |  |  |  |  |  |  |  |  |  |  |  |  |
Other investments (equity method) | $ 102.5 | $ 0.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Total as percent of investable assets | 56.40% | 53.40% |  |  |  |  |  |  |  |  |  |  |  |  |
Investable assets held by the Company | $ 7,800.0 | $ 8,700.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Investable assets held by insurance regulators | $ 503.2 | 458.7 |  |  |  |  |  |  |  |  |  |  |  |  |
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 | $ 1,311.4 | 1,268.4 |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held-in-trust | 1,336.4 | 1,350.9 |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory deposits | 6.0 | 6.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Deposit with states | 6.1 | 6.1 |  |  |  |  |  |  |  |  |  |  |  |  |
Rental and premises expenses | 27.3 | 26.3 |  |  |  |  |  |  |  |  |  |  |  |  |
Depreciation for fixed assets | 29.7 | 34.6 |  |  |  |  |  |  |  |  |  |  |  |  |
Accumulated depreciation | $ 175.8 | $ 146.2 |  |  |  |  |  |  |  |  |  |  |  |  |
Number of investments | Investment | 2 | 2 |  | 2 | 2 | 2 | 2 | 2 |  |  | 2 | 2 | 2 | 2 | 2
Us Multi Beneficiary Trust Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum trust fund | $ 20.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held-in-trust | 1,112.4 | $ 1,333.6 |  |  |  |  |  |  |  |  |  |  |  |  |
Us Surplus Lines Trust Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held-in-trust | $ 198.8 | 195.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Canadian Trust Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held-in-trust |  |  |  |  |  |  | $ 152.7 |  |  |  |  |  |  | $ 169.9 |
Australian Trust Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held-in-trust |  |  |  |  | $ 209.3 |  |  |  |  |  |  | $ 198.7 |  |  |
Swiss Trust Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held-in-trust | SFr |  |  |  |  |  | SFr 9.0 |  |  |  |  |  |  | SFr 9.8 |  |
Singapore Trust Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets held-in-trust |  |  |  |  |  |  |  | $ 135.9 |  |  |  |  |  |  | $ 120.6
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 | $ 647.8 | 895.5 |  |  |  |  |  |  |  |  |  |  |  |  |
Limited Partner | Real estate fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted assets | 0.0 | 100.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Other investments (equity method) | $ 102.5 | $ 0.0 |  |  |  |  |  |  | $ 13.8 | $ 86.2 |  |  |  |  |
Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Cash And Collateral [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business acquisition, share price | $ / shares |  |  | $ 42.75 |  |  |  |  |  |  |  |  |  |  |  |

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