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Unpaid Losses And Loss Adjustment Expenses (Prior Year Losses and Loss Adjustment Expenses Development by Product Line) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years | $ (38.6) | $ (167.4) | $ (479.8)
Insurance [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years | (104.7) | (142.9) | (506.3)
Insurance [Member] | General liability [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years |  |  | (139.7)
Insurance [Member] | General liability [Member] | U.S. and Bermuda |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years | 274.0 |  |
Insurance [Member] | General liability [Member] | International |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years | (36.5) |  |
Insurance [Member] | General liability [Member] | Short-Duration Insurance Contracts, Accident Years, all other |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years |  | (20.5) |
Insurance [Member] | General liability [Member] | Short-Duration Insurance Contracts, Accident Years 2016 to 2019 |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years |  | 61.6 |
Insurance [Member] | Professional liability [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years |  |  | (54.7)
Insurance [Member] | Professional liability [Member] | U.S. and Bermuda |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years | 56.8 |  |
Insurance [Member] | Professional liability [Member] | International |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years | (102.2) |  |
Insurance [Member] | Professional liability [Member] | Short-Duration Insurance Contracts, Accident Years 2018 and 2019 |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years |  | 121.0 |
Insurance [Member] | Professional liability [Member] | Short-Duration Insurance Contracts, Accident Years, all other |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years |  | (91.1) |
Insurance [Member] | Property [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years | (95.1) | (48.1) | (96.5)
Insurance [Member] | Marine and energy [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years | (76.5) |  | (60.0)
Insurance [Member] | Personal Lines |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years | (42.8) |  |
Insurance [Member] | Workers' compensation [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years | (36.5) | (62.1) | (79.0)
Insurance [Member] | Other products [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years | (45.9) | (23.7) | (76.4)
Insurance [Member] | Programs [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years |  | (48.3) |
Insurance [Member] | Credit and surety [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years |  | (31.7) |
Reinsurance [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years | 57.1 | (26.1) | 19.9
Reinsurance [Member] | General liability [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years | 95.5 |  | (19.2)
Reinsurance [Member] | Professional liability [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years |  |  | 29.2
Reinsurance [Member] | Property [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years | (21.6) | (29.2) | 35.0
Reinsurance [Member] | Workers' compensation [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years | (14.9) |  |
Reinsurance [Member] | Other products [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years | (67.0) | (27.1) | (8.5)
Reinsurance [Member] | Premium adjustments |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years | 11.4 | 53.1 |
Reinsurance [Member] | Credit and surety [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years |  | (22.9) | (16.6)
Reinsurance [Member] | Public entity |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years | 53.7 |  |
Other [Member] | Other underwriting [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years | $ 9.0 | $ 1.6 | $ 6.6

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