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Significant Transactions - Ark (Details) - USD ($) $ in Millions |  |  | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Jan. 01, 2021 | Oct. 01, 2020 | Mar. 31, 2021 | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019 | Sep. 30, 2021
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |
Change in fair value of contingent consideration |  |  |  | $ 1.0 | $ (3.3) | $ 2.1 |
Other Operations |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |
Cash pre-funded/placed in escrow for Ark Transaction |  |  |  | 0.0 | 646.3 |  |
Goodwill and other intangible assets |  |  |  | 39.1 | 36.4 |  |
Ark |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |
Cash pre-funded/placed in escrow for Ark Transaction |  |  |  |  | 366.3 |  |
Goodwill and other intangible assets |  |  |  | 292.5 | 0.0 |  |
Contingent consideration |  |  |  | 28.0 | 0.0 |  |
Change in fair value of contingent consideration |  |  |  | 5.5 |  |  |
Ark | In-force Business Acquired |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |
Amortization |  |  |  | $ 64.2 |  |  |
Ark | Lloyd |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |
Cash pre-funded/placed in escrow for Ark Transaction |  |  |  |  | $ 280.0 |  |
Ark |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |
Merger agreement, equity capital contributed | $ 605.4 | $ 605.4 |  |  |  |  |
Equity capital, pre money valuation amount |  | 300.0 |  |  |  |  |
Additional equity purchase from shareholders |  | 40.9 |  |  |  |  |
Investment, potential additional contribution of equity capital | $ 200.0 | $ 200.0 |  |  |  |  |
Ownership interest after all transactions (as a percent) | 72.00% |  |  |  |  |  |
Ownership interest on fully-diluted basis (as a percent) | 63.00% |  |  |  |  |  |
Percent of shares outstanding | 12.50% |  |  | 12.50% |  |  |
Assets acquired | $ 2,500.0 |  |  |  |  |  |
Goodwill and other intangible assets | 292.5 |  |  |  |  |  |
Liabilities acquired | 1,700.0 |  |  |  |  |  |
Contingent consideration | 22.5 |  |  |  |  |  |
Noncontrolling interest acquired | $ 220.2 |  |  |  |  |  |
Transaction costs |  |  | $ 25.3 |  |  |  |
Ark | Group Ark Insurance Limited | Unsecured Subordinated Debt |  |  |  |  |  |  |
Other Significant Noncash Transactions [Line Items] |  |  |  |  |  |  |
Notes issued |  |  |  |  |  |  | $ 163.3

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