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Significant Transactions - Schedule of Assets Acquired and Liabilities Assumed, Ark (Details) - USD ($) $ in Millions | Jan. 01, 2021 | Oct. 01, 2020 | Dec. 31, 2021 | Dec. 31, 2020
--- | --- | --- | --- | ---
Other Significant Transactions [Line Items] |  |  |  |
Goodwill |  |  | $ 645.5 | $ 525.5
Ark |  |  |  |
Other Significant Transactions [Line Items] |  |  |  |
Investments | $ 594.3 |  |  |
Cash | 52.0 |  |  |
Reinsurance recoverables | 433.4 |  |  |
Insurance premiums receivable | 236.7 |  |  |
Ceded unearned premiums | 170.2 |  |  |
Value of in-force business acquired | 71.7 |  |  |
Other assets | 88.9 |  |  |
Loss and loss adjustment expense reserves | (696.0) |  |  |
Unearned insurance premiums | (326.1) |  |  |
Debt | (46.4) |  |  |
Ceded reinsurance payable | (528.3) |  |  |
Other liabilities | (25.9) |  |  |
Net tangible assets acquired | 24.5 |  |  |
Goodwill | 116.8 |  |  |
Other intangible assets - syndicate underwriting capacity | 175.7 |  |  |
Deferred tax liability on other intangible assets | (33.4) |  |  |
Net assets acquired | 283.6 |  |  |
Merger agreement, equity capital contributed | $ 605.4 | $ 605.4 |  |

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