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.