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Reserves for Losses and Adjustment Expenses - Reconciliation of Beginning and Ending Consolidated Loss and Loss Adjustment Expenses (LAE) Reserves (Details) - USD ($) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Mar. 02, 2020 | Dec. 31, 2017
Insurance [Abstract] |  |  |  |  |  |
Provision for losses and LAE, beginning balance |  | $ 6,951.8 | $ 7,074.2 | $ 6,749.5 |  |
Less reinsurance recoverable |  |  | (2,319.8) | (2,077.6) |  | $ (1,515.2)
Net losses and LAE reserves, beginning balance |  | 4,632.0 | 4,996.6 | 5,234.3 |  |
Net loss and LAE expenses (disposed) (1) |  | (818.5) | 0.0 | 0.0 |  |
Movement in net provision for losses and LAE for claims incurred: |  |  |  |  |  |
Current year |  | 1,841.7 | 1,620.2 | 1,684.1 |  |
Prior years |  | (0.9) | 59.5 | (111.1) |  |
Total incurred |  | 1,840.8 | 1,679.7 | 1,573.0 |  |
Losses and LAE payments for claims incurred: |  |  |  |  |  |
Current year |  | (404.9) | (428.5) | (285.7) |  |
Prior years |  | (1,359.7) | (1,694.1) | (1,441.0) |  |
Total paid |  | (1,764.6) | (2,122.6) | (1,726.7) |  |
Foreign exchange losses/(gains) |  | 80.4 | 78.3 | (84.0) |  |
Net losses and LAE reserves, ending balance |  | 3,970.1 | 4,632.0 | 4,996.6 |  |
Plus reinsurance recoverable on unpaid losses at the end of the year |  | 3,195.2 | 2,319.8 | 2,077.6 |  |
Provision for losses and LAE at the end of the year |  | 7,165.3 | 6,951.8 | 7,074.2 |  |
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items] |  |  |  |  |  |
Net loss and LAE expenses (disposed) (1) |  | (818.5) | 0.0 | 0.0 |  |
Insurance |  |  |  |  |  |
Insurance [Abstract] |  |  |  |  |  |
Provision for losses and LAE, beginning balance |  | 2,026.1 | 2,153.0 |  |  |
Net losses and LAE reserves, beginning balance |  | 2,026.1 | 2,153.0 |  |  |
Movement in net provision for losses and LAE for claims incurred: |  |  |  |  |  |
Prior years |  | (35.2) | (114.4) |  |  |
Losses and LAE payments for claims incurred: |  |  |  |  |  |
Net losses and LAE reserves, ending balance |  | 1,874.4 | 2,026.1 | 2,153.0 |  |
Provision for losses and LAE at the end of the year |  | 1,874.4 | $ 2,026.1 | $ 2,153.0 |  |
Surety Product Line | Insurance |  |  |  |  |  |
Insurance [Abstract] |  |  |  |  |  |
Net loss and LAE expenses (disposed) (1) |  | 42.0 |  |  |  |
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items] |  |  |  |  |  |
Net loss and LAE expenses (disposed) (1) |  | 42.0 |  |  |  |
Accident and Health Insurance Product Line |  |  |  |  |  |
Insurance [Abstract] |  |  |  |  |  |
Net loss and LAE expenses (disposed) (1) |  | 6.5 |  |  |  |
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items] |  |  |  |  |  |
Net loss and LAE expenses (disposed) (1) |  | 6.5 |  |  |  |
Enstar |  |  |  |  |  |
Insurance [Abstract] |  |  |  |  |  |
Net loss and LAE expenses (disposed) (1) |  | 770.0 |  |  |  |
Losses and LAE payments for claims incurred: |  |  |  |  |  |
Plus reinsurance recoverable on unpaid losses at the end of the year | [1] |  |  |  | $ 770.0 |
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items] |  |  |  |  |  |
Net loss and LAE expenses (disposed) (1) |  | $ 770.0 |  |  |  |
[1] | Included within reinsurance recoverable for unpaid losses are $770.0 million of recoveries associated with the purchase of an adverse development cover.

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