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Significant Transactions - Schedule of Assets Acquired and Liabilities Assumed, Ark (Details) - USD ($) $ in Millions | Mar. 31, 2021 | Jan. 01, 2021 | Dec. 31, 2020
--- | --- | --- | ---
Other Significant Transactions [Line Items] |  |  |
Goodwill | $ 613.4 |  | $ 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 |

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