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Unpaid losses and loss expenses (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Jul. 01, 2023 | Dec. 31, 2020 | Dec. 31, 2004
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | $ (773) | $ (876) |  | $ (926) |  |  |  |
Prior Period Development, net Adjustments |  | $ 83 | $ 232 |  | $ 30 |  |  |  |
Prior Period Development Percentage Opening Net Unpaid Reserves | [1] | 1.30% | 1.60% |  | 1.80% |  |  |  |
Incurred activity |  | $ 24,100 | $ 22,572 |  | $ 21,030 |  |  |  |
Liability for Claims and Claims Adjustment Expense |  | 80,122 | 75,747 |  | 72,330 |  |  | $ 67,192 |
Net losses and loss expenses paid in Prior Year |  | 12,763 | 12,206 |  | 10,048 |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net |  | 62,238 | 58,661 |  | 56,198 |  |  | $ 52,616 |
Prior Year Claims and Claims Adjustment Expense |  | (882) |  |  |  |  |  |  |
Prior Year Claims and Claims Adjustment Expense, net of PPD related to A&H |  | (856) | (1,108) | [2] | (956) | [2] |  |  |
Huatai Group [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense |  |  |  |  |  |  | $ 831 |  |
Brandywine Run-off [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Incurred activity |  | 712 |  |  |  |  |  |  |
Reinsurance coverage to Century provided by ACE INA under XOL |  | 800 |  |  |  |  |  |  |
Statutory capital and surplus |  | 25 |  |  |  |  |  |  |
Dividend retention fund established by INA Financial Corporation |  | 50 |  |  |  |  |  |  |
Required minimum balance under the dividend retention fund |  | 50 |  |  |  |  |  |  |
Contributions to the dividend retention fund |  | 75 | 75 |  |  |  |  |  |
Minimum contribution from the dividend retention fund to Century not required for XOL agreement |  | 200 |  |  |  |  |  |  |
Dividend Retention Fund Contribution to XOL |  | 75 | 106 |  |  |  |  |  |
Aggregate reinsurance balances ceded by active ACE companies to Century |  | 1,800 | 1,900 |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense |  | 1,700 | 2,100 |  |  |  |  |  |
Surplus note |  |  |  |  |  |  |  |  | $ 100
Century X O L Reinsurance Coverage, Statutory-Basis Remaining Limit |  | $ 88 |  |  |  |  |  |  |
Westchester and Brandywine Run-off [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
NICO pro-rata share of reinsurance protection (percent) |  | 75.00% |  |  |  |  |  |  |
NICO retention for losses and loss expenses incurred on or before 12/31/1996 |  | $ 721 |  |  |  |  |  |  |
NICO reinsurance protection on losses and loss expenses incurred on or before 12/31/1996, net of retenion |  | 1,000 |  |  |  |  |  |  |
NICO reinsurance protection on losses and loss expenses |  | 337 |  |  |  |  |  |  |
Long Tail [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | 148 | 58 |  | (44) |  |  |  |
Short Tail [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | (921) | (934) |  | (882) |  |  |  |
North America Commercial P&C Insurance - Workers' Compensation [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | 92 |  |  |  |  |  |  |
Boy Scouts of America Agreement in Principle [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense |  | 500 | 800 |  |  |  |  |  |
Net losses and loss expenses paid in Prior Year |  | 300 |  |  |  |  |  |  |
North America Commercial P&C Insurance [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | (494) | $ (562) |  | $ (762) |  |  |  |
Prior Period Development, net Adjustments | [3] | $ 98 |  |  |  |  |  |  |
Prior Period Development Percentage Opening Net Unpaid Reserves | [1] | 0.80% | 1.00% |  | 1.40% |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | $ (592) |  |  |  |  |  |  |
North America Commercial P&C Insurance [Member] | Long Tail [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | (86) | $ (229) |  | $ (482) |  |  |  |
Prior Period Development, net Adjustments |  | 90 |  |  |  |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (176) |  |  |  |  |  |  |
North America Commercial P&C Insurance [Member] | Short Tail [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | (408) | (333) |  | (280) |  |  |  |
Prior Period Development, net Adjustments |  | 8 |  |  |  |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (416) |  |  |  |  |  |  |
North America Commercial P&C Insurance [Member] | Commercial Excess and Umbrella [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  |  | 96 |  |  |  |  |  |
North America Commercial P&C Insurance [Member] | Workers' Compensation Insurance [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | (387) |  |  |  |  |  |  |
North America Commercial P&C Insurance [Member] | Workers' Compensation Insurance [Member] | Long Tail [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  |  | (496) |  |  |  |  |  |
North America Commercial P&C Insurance [Member] | Property and Inland Marine |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | (248) |  |  |  |  |  |  |
North America Commercial P&C Insurance [Member] | Property and Inland Marine | Short Tail [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  |  | (206) |  |  |  |  |  |
North America Commercial P&C Insurance [Member] | Auto Liability |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | 129 | 177 |  |  |  |  |  |
North America Commercial P&C Insurance [Member] | Medical Portfolios [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  |  | 82 |  |  |  |  |  |
North America Personal P&C Insurance [Member] | Homeowners and Valuables [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | (244) |  |  |  |  |  |  |
North America Personal P&C Insurance [Member] | Personal Excess Liability |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | 145 |  |  |  |  |  |  |
North America Personal P&C Insurance [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | $ (134) | $ (186) |  | $ (305) |  |  |  |
Prior Period Development Percentage Opening Net Unpaid Reserves | [1] | 0.20% | 0.30% |  | 0.60% |  |  |  |
North America Personal P&C Insurance [Member] | Long Tail [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | $ 0 | $ 0 |  | $ 0 |  |  |  |
North America Personal P&C Insurance [Member] | Short Tail [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | (134) | (186) |  | (305) |  |  |  |
Prior Period Development, net Adjustments |  | (2) |  |  |  |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (132) |  |  |  |  |  |  |
North America Agricultural Insurance [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | $ (18) | $ (61) |  | $ 10 |  |  |  |
Prior Period Development Percentage Opening Net Unpaid Reserves | [1] | 0.00% | 0.10% |  | 0.00% |  |  |  |
North America Agricultural Insurance [Member] | Long Tail [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | $ 0 | $ 0 |  | $ 0 |  |  |  |
North America Agricultural Insurance [Member] | Short Tail [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | (18) | (61) |  | 10 |  |  |  |
Prior Period Development, net Adjustments |  | 6 |  |  |  |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (24) |  |  |  |  |  |  |
Overseas General Insurance [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | (376) | $ (448) |  | $ (441) |  |  |  |
Prior Period Development, net Adjustments |  | $ 0 |  |  |  |  |  |  |
Prior Period Development Percentage Opening Net Unpaid Reserves | [1] | 0.60% | 0.80% |  | 0.80% |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | $ (376) |  |  |  |  |  |  |
Overseas General Insurance [Member] | Long Tail [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | (50) | $ (65) |  | $ (106) |  |  |  |
Prior Period Development, net Adjustments |  | 0 |  |  |  |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (50) |  |  |  |  |  |  |
Overseas General Insurance [Member] | Short Tail [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | (326) | (383) |  | (335) |  |  |  |
Prior Period Development, net Adjustments |  | 0 |  |  |  |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (326) |  |  |  |  |  |  |
Overseas General Insurance [Member] | Accident and Health [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  |  | (105) |  |  |  |  |  |
Overseas General Insurance [Member] | Property and Casualty [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  |  | (100) |  |  |  |  |  |
Overseas General Insurance [Member] | Property and Inland Marine |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | (253) |  |  |  |  |  |  |
Overseas General Insurance [Member] | Professional Lines |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | $ (77) |  |  |  |  |  |  |
Overseas General Insurance - Casualty [Member] | Europe [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Loss by Geographic Percentage |  | 46.00% |  |  |  |  |  |  |
Global Reinsurance Non-Casualty [Member] | Accident years 2013 and after |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Loss by Geographic Percentage |  | 87.00% |  |  |  |  |  |  |
Global Reinsurance Non-Casualty [Member] | Accident years 2018 to 2022 |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Loss by Geographic Percentage |  | 93.00% |  |  |  |  |  |  |
Global Reinsurance Non-Casualty [Member] | Accident years 2013-2017 |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Loss by Geographic Percentage |  | 77.00% |  |  |  |  |  |  |
Global Reinsurance [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | $ (28) | $ 22 |  | $ 3 |  |  |  |
Prior Period Development, net Adjustments |  | $ 7 |  |  |  |  |  |  |
Prior Period Development Percentage Opening Net Unpaid Reserves | [1] | 0.00% | 0.00% |  | 0.00% |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | $ (35) |  |  |  |  |  |  |
Global Reinsurance [Member] | Long Tail [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | 7 | $ (7) |  | $ (25) |  |  |  |
Prior Period Development, net Adjustments |  | 0 |  |  |  |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | 7 |  |  |  |  |  |  |
Global Reinsurance [Member] | Short Tail [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | (35) | 29 |  | 28 |  |  |  |
Prior Period Development, net Adjustments |  | 7 |  |  |  |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | (42) |  |  |  |  |  |  |
Corporate Segment [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | $ 277 | $ 359 |  | $ 569 |  |  |  |
Prior Period Development Percentage Opening Net Unpaid Reserves | [1] | 0.50% | 0.60% |  | 1.10% |  |  |  |
Corporate Segment [Member] | Long Tail [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | $ 277 | $ 359 |  | $ 569 |  |  |  |
Prior Period Development, net Adjustments |  | 0 |  |  |  |  |  |  |
Prior Year Claims and Claims Adjustment Expense |  | 277 |  |  |  |  |  |  |
Corporate Segment [Member] | Short Tail [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | 0 | 0 |  | 0 |  |  |  |
Corporate Segment [Member] | Other [Member] | Long Tail [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | (49) | 155 |  | 417 |  |  |  |
Corporate Segment [Member] | Asbestos & Environmental [Member] | Long Tail [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | 99 | $ 61 |  | $ 52 |  |  |  |
North America Commercial P&C Insurance [Member] | Commercial Excess and Umbrella [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | 193 |  |  |  |  |  |  |
North America Commercial P&C Insurance [Member] | Surety Product Line [Member] |  |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |  |
Net Prior Period Development |  | $ (101) |  |  |  |  |  |  |
[1] | Calculated based on the beginning of period consolidated net unpaid losses and loss expenses.
[2] | Relates to prior period loss reserve development only and excludes prior period development related to reinstatement premiums, expense adjustments, earned premiums, and A&H long-duration lines totaling $83 million, $232 million, and $30 million for 2023, 2022, and 2021, respectively.
[3] | Includes premium returns associated with our Alternative Risk Solutions business, which is excluded from the triangles.

X
- Definition The aggregate reinsurance balances ceded by active ACE companies to Century Indemnity Company. + References No definition available. + Details
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- Definition Amount of excess of loss reinsurance provided to Century from an ACE INA subsidiary. + References No definition available. + Details
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- Definition Dividend retention fund created by INA Financial Corporation established in respect of the restructuring of the Brandywine operations. + References No definition available. + Details
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- Definition Percent pro-rata share of reinsurance protection on losses and loss expenses incurred on or before December 31, 1992. + References No definition available. + Details
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