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Unpaid losses and loss expenses (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2004
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  | $ (876) | $ (926) | $ (395) |  |
Prior Period Development, net Adjustments |  | $ 277 | $ 60 | $ 19 |  |
Prior Period Development Percentage Opening Net Unpaid Reserves | [1] | 1.50% | 1.70% | 0.80% |  |
Incurred activity |  | $ 23,342 | $ 21,980 | $ 21,710 |  |
Liability for Claims and Claims Adjustment Expense |  | 76,323 | 72,943 | 67,811 | $ 62,690 |
Net losses and loss expenses paid in Prior Year |  | 12,206 | 10,048 | 9,652 |  |
Brandywine Run-off [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Incurred activity |  | 669 |  |  |  |
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 | 50 |  |  |
Minimum contribution from the dividend retention fund to Century not required for XOL agreement |  | 200 |  |  |  |
Dividend Retention Fund Contribution to XOL |  | 106 | 18 |  |  |
Aggregate reinsurance balances ceded by active ACE companies to Century |  | 1,900 | 1,800 |  |  |
Liability for Claims and Claims Adjustment Expense |  | 2,100 | 2,200 |  |  |
Surplus note |  |  |  |  |  | $ 100
Century X O L Reinsurance Coverage, Statutory-Basis Remaining Limit |  | $ 131 |  |  |  |
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 |  | 347 |  |  |  |
Long Tail [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  | 58 | (44) | (313) |  |
Short Tail [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  | (934) | (882) | (82) |  |
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 |  |  |
Reinsurance Recoverable and Previously Carried Reserves |  | 425 |  |  |  |
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 |  | (562) | $ (762) | $ (702) |  |
Prior Period Development, net Adjustments | [2] | $ 112 |  |  |  |
Prior Period Development Percentage Opening Net Unpaid Reserves | [1] | 1.00% | 1.40% | 1.40% |  |
North America Commercial P&C Insurance [Member] | Long Tail [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  | $ (229) | $ (482) | $ (672) |  |
Prior Period Development, net Adjustments |  | 85 |  |  |  |
North America Commercial P&C Insurance [Member] | Short Tail [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  | (333) | (280) | (30) |  |
Prior Period Development, net Adjustments |  | 27 |  |  |  |
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] | Management Liability [Member] | Long Tail [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  |  | (278) |  |  |
North America Commercial P&C Insurance [Member] | Workers' Compensation Insurance [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  | (496) |  |  |  |
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 |  |  | (260) |  |  |
North America Commercial P&C Insurance [Member] | Property and Inland Marine |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  | (206) |  |  |  |
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 |  |  | (164) |  |  |
North America Commercial P&C Insurance [Member] | Auto Liability |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  | 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 Commercial P&C Insurance [Member] | COVID-19 [Member] | Management Liability [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  |  | (303) |  |  |
North America Commercial P&C Insurance [Member] | COVID-19 [Member] | Management Liability [Member] | Long Tail [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  |  | (256) |  |  |
North America Personal P&C Insurance [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  | $ (186) | $ (305) | $ 63 |  |
Prior Period Development Percentage Opening Net Unpaid Reserves | [1] | 0.30% | 0.60% | 0.10% |  |
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 |  | (186) | (305) | 63 |  |
Prior Period Development, net Adjustments |  | 0 |  |  |  |
North America Agricultural Insurance [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  | $ (61) | $ 10 | $ (10) |  |
Prior Period Development Percentage Opening Net Unpaid Reserves | [1] | 0.10% | 0.00% | 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 |  | (61) | 10 | (10) |  |
Prior Period Development, net Adjustments |  | 166 |  |  |  |
Overseas General Insurance [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  | (448) | $ (441) | $ (150) |  |
Prior Period Development, net Adjustments |  | $ 0 |  |  |  |
Prior Period Development Percentage Opening Net Unpaid Reserves | [1] | 0.80% | 0.80% | 0.30% |  |
Overseas General Insurance [Member] | Long Tail [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  | $ (65) | $ (106) | $ (49) |  |
Prior Period Development, net Adjustments |  | 0 |  |  |  |
Overseas General Insurance [Member] | Short Tail [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  | (383) | (335) | (101) |  |
Prior Period Development, net Adjustments |  | 0 |  |  |  |
Overseas General Insurance [Member] | Financial [Member] | Long Tail [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  |  | 90 |  |  |
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] | Accident and Health [Member] | Short Tail [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  |  | (111) |  |  |
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] | COVID-19 [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  |  | (127) |  |  |
Overseas General Insurance [Member] | COVID-19 [Member] | Financial [Member] | Long Tail [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  |  | (104) |  |  |
Overseas General Insurance - Casualty [Member] | Europe [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Loss by Geographic Percentage |  | 45.00% |  |  |  |
Global Reinsurance Non-Casualty [Member] | Accident years 2013 and after |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Loss by Geographic Percentage |  | 85.00% |  |  |  |
Global Reinsurance Non-Casualty [Member] | Accident years 2018 to 2022 |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Loss by Geographic Percentage |  | 91.00% |  |  |  |
Global Reinsurance Non-Casualty [Member] | Accident years 2013-2017 |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Loss by Geographic Percentage |  | 79.00% |  |  |  |
Global Reinsurance [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  | $ 22 | $ 3 | $ (29) |  |
Prior Period Development, net Adjustments |  | $ (1) |  |  |  |
Prior Period Development Percentage Opening Net Unpaid Reserves | [1] | 0.00% | 0.00% | 0.10% |  |
Global Reinsurance [Member] | Long Tail [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  | $ (7) | $ (25) | $ (25) |  |
Prior Period Development, net Adjustments |  | 0 |  |  |  |
Global Reinsurance [Member] | Short Tail [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  | 29 | 28 | (4) |  |
Prior Period Development, net Adjustments |  | (1) |  |  |  |
Corporate Segment [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  | $ 359 | $ 569 | $ 433 |  |
Prior Period Development Percentage Opening Net Unpaid Reserves | [1] | 0.60% | 1.10% | 0.90% |  |
Corporate Segment [Member] | Long Tail [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net Prior Period Development |  | $ 359 | $ 569 | $ 433 |  |
Prior Period Development, net Adjustments |  | 0 |  |  |  |
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 |  | $ 155 | $ 417 | $ 254 |  |
[1] | Calculated based on the beginning of period consolidated net unpaid losses and loss expenses.
[2] | Includes premium returns associated with our Alternative Risk Solutions business, which is excluded from the triangles.

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