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Unpaid losses and loss expenses (Narrative) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2004
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development $ (926) $ (395) $ (792)    
Prior Period Development, net Adjustments $ 60 $ 19 $ 53    
Prior Period Development Percentage Opening Net Unpaid Reserves [1] 1.70% 0.80% 1.60%    
Incurred activity $ 21,980 $ 21,710 $ 18,730    
Liability for Claims and Claims Adjustment Expense 72,943 67,811 62,690 $ 62,960  
Brandywine Run-off [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Incurred activity 745        
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 50 250      
Minimum contribution from the dividend retention fund to Century not required for XOL agreement 200        
Dividend Retention Fund Contribution to XOL 18 302      
Aggregate reinsurance balances ceded by active ACE companies to Century 1,800 1,600      
Liability for Claims and Claims Adjustment Expense 2,200 2,100      
Surplus note         $ 100
Century X O L Reinsurance Coverage, Statutory-Basis Remaining Limit $ 55        
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 359        
Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development (44) (313) (642)    
Short Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development (882) (82) (150)    
Boy Scouts of America Agreement in Principle [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Liability for Claims and Claims Adjustment Expense 800        
Reinsurance Recoverable and Previously Carried Reserves 425        
North America Commercial P&C Insurance [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development (762) $ (702) $ (649)    
Prior Period Development, net Adjustments [2] $ 79        
Prior Period Development Percentage Opening Net Unpaid Reserves [1] 1.40% 1.40% 1.30%    
North America Commercial P&C Insurance [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development $ (482) $ (672) $ (668)    
Prior Period Development, net Adjustments 69        
North America Commercial P&C Insurance [Member] | Short Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development (280) (30) 19    
Prior Period Development, net Adjustments 10        
North America Commercial P&C Insurance [Member] | Multi-Line [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development 46 26      
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   (363)      
North America Commercial P&C Insurance [Member] | Auto Liability | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development   77      
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] | Accident years 2014 and prior [Member] | Commercial Excess and Umbrella [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development   (83)      
North America Commercial P&C Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Foreign Casualty Line [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development   (40)      
North America Commercial P&C Insurance [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member] | Surety Product Line [Member] | Short Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development   (31)      
North America Commercial P&C Insurance [Member] | Accident years 2017 and prior [Member] | Multi-Line [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development (43)        
North America Commercial P&C Insurance [Member] | Accident years 2017 and prior [Member] | Environmental lines [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development (40)        
North America Commercial P&C Insurance [Member] | Accident years 2016 and prior [Member] | Professional Errors and Omissions Insurance [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development   (67)      
North America Commercial P&C Insurance [Member] | Accident years 2016 and prior [Member] | Multi-Line [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development   (41)      
North America Commercial P&C Insurance [Member] | Accident years 2016 and prior [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] | Accident years 2016 and prior [Member] | Environmental lines [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development   (43)      
North America Commercial P&C Insurance [Member] | Accident Year 2015 and prior [Member] | Management Liability [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development   (231)      
North America Commercial P&C Insurance [Member] | Accident Year 2017 to 2019 [Member] | Auto Liability | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development 71        
North America Commercial P&C Insurance [Member] | Accident years 2016 - 2019 [Member] | Commercial Excess and Umbrella [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development 32        
North America Commercial P&C Insurance [Member] | Accident years 2016 - 2019 [Member] | Medical Portfolios [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development   64      
North America Commercial P&C Insurance [Member] | Accident years 2018 - 2019 [Member] | Accident and Health [Member] | Short Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development   (37)      
North America Commercial P&C Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2019 [Member] | Workers' Compensation Insurance [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development   (62)      
North America Commercial P&C Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2019 [Member] | Property and Inland Marine | Short Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development   21      
North America Commercial P&C Insurance [Member] | Accident years 2017 - 2019 | General Liability Coverages | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development   48      
North America Commercial P&C Insurance [Member] | Accident Years 2019 - 2020 | Accident and Health [Member] | Short Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development (45)        
North America Commercial P&C Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2020 | Surety Product Line [Member] | Short Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development (35)        
North America Commercial P&C Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2020 | 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] | Short-Duration Insurance Contract, Accident Year 2020 | Workers' Compensation Insurance [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development (36)        
North America Commercial P&C Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2020 | 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 Commercial P&C Insurance [Member] | Accident years 2016, 2017, and 2020 [Member] | Professional Errors and Omissions Insurance [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development (59)        
North America Commercial P&C Insurance [Member] | Accident years 2004 and prior [Member] | Other [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development 26        
North America Commercial P&C Insurance [Member] | Accident years 2017 and 2018 [Member] | Medical Portfolios [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development 66        
North America Commercial P&C Insurance [Member] | Accident Years 2019 and 2020 | 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] | Accident years 2018 - 2020 [Member] | Surety Product Line [Member] | Short Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development (89)        
North America Commercial P&C Insurance [Member] | Accident years 2018 - 2020 [Member] | First Party Cyber risk | Short Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development 42        
North America Personal P&C Insurance [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development $ (305) $ 63 $ (95)    
Prior Period Development Percentage Opening Net Unpaid Reserves [1] 0.60% 0.10% 0.20%    
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 (305) 63 (95)    
Prior Period Development, net Adjustments (1)        
North America Personal P&C Insurance [Member] | Personal Excess [Member] | Short Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development   (22)      
North America Personal P&C Insurance [Member] | Accident Year 2017 to 2019 [Member] | Homeowners and Valuables [Member] | Short Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development   84      
North America Personal P&C Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2020 | Homeowners and Valuables [Member] | Short Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development (253)        
North America Personal P&C Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2020 | Auto Liability | Short Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development (43)        
North America Agricultural Insurance [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development $ 10 $ (10) $ (80)    
Prior Period Development Percentage Opening Net Unpaid Reserves [1] 0.00% 0.00% 0.20%    
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 10 (10) (80)    
Prior Period Development, net Adjustments (28)        
Overseas General Insurance [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development (441) $ (150) $ (92)    
Prior Period Development, net Adjustments $ 7        
Prior Period Development Percentage Opening Net Unpaid Reserves [1] 0.80% 0.30% 0.20%    
Overseas General Insurance [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development $ (106) $ (49) $ (68)    
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 (335) (101) (24)    
Prior Period Development, net Adjustments 7        
Overseas General Insurance [Member] | Casualty Lines [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development (67) (94)      
Overseas General Insurance [Member] | Financial [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development (14) 80      
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] | Accident years 2017 and prior [Member] | Casualty Lines [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development (95)        
Overseas General Insurance [Member] | Accident years 2016 and prior [Member] | Casualty Lines [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development   (143)      
Overseas General Insurance [Member] | Accident Year 2015 and prior [Member] | Financial [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development   (61)      
Overseas General Insurance [Member] | Accident Year 2017 to 2019 [Member] | Casualty Lines [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development   49      
Overseas General Insurance [Member] | Accident years 2016 - 2019 [Member] | Financial [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development   141      
Overseas General Insurance [Member] | Accident years 2018 - 2019 [Member] | Accident and Health [Member] | Short Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development   (21)      
Overseas General Insurance [Member] | Accident years 2018 - 2019 [Member] | Political Risk | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development   (22)      
Overseas General Insurance [Member] | Accident years 2012 through 2019 | Marine [Member] | Short Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development   (69)      
Overseas General Insurance [Member] | Accident Years 2019 - 2020 | 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] | Accident Years 2019 - 2020 | Property and Casualty [Member] | Short Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development (67)        
Overseas General Insurance [Member] | Accident Years 2019 - 2020 | Consumer [Member] | Short Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development (71)        
Overseas General Insurance [Member] | Short-Duration Insurance Contract, Accident Year 2020 | 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 [Member] | Short-Duration Insurance Contract, Accident Year 2020 | COVID-19 [Member] | Accident and Health [Member] | Short Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development (20)        
Overseas General Insurance [Member] | Accident years 2018 - 2020 [Member] | Casualty Lines [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development 28        
Overseas General Insurance [Member] | Accident years 2018 - 2020 [Member] | Marine [Member] | Short Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development (62)        
Overseas General Insurance [Member] | Accident years 2011 - 2020 [Member] | Financial [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development $ 90        
Overseas General Insurance - Casualty [Member] | Europe [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Loss by Geographic Percentage 46.00%        
Overseas General Insurance - Non-Casualty [Member] | Europe and Asia [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Loss by Geographic Percentage 33.00%        
Global Reinsurance Non-Casualty [Member] | accident year 2012 and after          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Loss by Geographic Percentage 79.00%        
Global Reinsurance Non-Casualty [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Loss by Geographic Percentage 54.00%        
Global Reinsurance Non-Casualty [Member] | Accident year 2013 to 2021          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Loss by Geographic Percentage 83.00%        
Global Reinsurance [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development $ 3 $ (29) $ (29)    
Prior Period Development, net Adjustments $ 3        
Prior Period Development Percentage Opening Net Unpaid Reserves [1] 0.00% 0.10% 0.10%    
Global Reinsurance [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development $ (25) $ (25) $ (59)    
Prior Period Development, net Adjustments 3        
Global Reinsurance [Member] | Short Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development 28 (4) 30    
Prior Period Development, net Adjustments 0        
Global Reinsurance [Member] | Short-Duration Insurance Contract, Accident Year 2020 | Property catastrophe [Member] | Short Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development 34        
Corporate Segment [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development $ 569 $ 433 $ 153    
Prior Period Development Percentage Opening Net Unpaid Reserves [1] 1.10% 0.90% 0.30%    
Corporate Segment [Member] | Discontinued Operations [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development $ 43 $ 38      
Corporate Segment [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development 569 433 $ 153    
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] | Other [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development 14 27      
Corporate Segment [Member] | Other [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development 417 254      
Corporate Segment [Member] | Asbestos & Environmental [Member] | Long Tail [Member]          
Liability for Claims and Claims Adjustment Expense [Line Items]          
Net Prior Period Development $ 83 $ 106      
[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.