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.