Unpaid losses and loss expenses (Narrative) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2024	Dec. 31, 2023		Dec. 31, 2022		Jul. 01, 2023	Dec. 31, 2021	Dec. 31, 2004
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ (856)	$ (773)		$ (876)
Prior Period Development, net Adjustments		$ 119	$ 83		$ 232
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	1.40%	1.30%		1.60%
Incurred activity		$ 26,022	$ 24,100		$ 22,572
Liability for Claims and Claims Adjustment Expense		84,004	80,122		75,747			$ 72,330
Net losses and loss expenses paid in Prior Year		12,822	12,763		12,206
Liability for Unpaid Claims and Claims Adjustment Expense, Net		66,270	62,238		58,661			$ 56,198
Prior Year Claims and Claims Adjustment Expense	[2]	(991)
Prior Year Claims and Claims Adjustment Expense, net of PPD related to A&H		(975)	(856)	[3]	(1,108)	[3]
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		764
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		93	75
Minimum contribution from the dividend retention fund to Century not required for XOL agreement		200
Dividend Retention Fund Contribution to XOL		93	75
Aggregate reinsurance balances ceded by active ACE companies to Century		1,900	1,800
Liability for Claims and Claims Adjustment Expense		1,600	1,700
Surplus note									$ 100
Century X O L Reinsurance Coverage, Statutory-Basis Remaining Limit		$ 36
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		332
Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		288	148		58
Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(1,144)	(921)		(934)
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		89
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		(428)	$ (494)		$ (562)
Prior Period Development, net Adjustments	[4]	$ 65
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	0.70%	0.80%		1.00%
Prior Year Claims and Claims Adjustment Expense		$ (493)
North America Commercial P&C Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		18	$ (86)		$ (229)
Prior Period Development, net Adjustments		71
Prior Year Claims and Claims Adjustment Expense		(53)
North America Commercial P&C Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(446)	(408)		(333)
Prior Period Development, net Adjustments		(6)
Prior Year Claims and Claims Adjustment Expense		(440)
North America Personal P&C Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ (305)	$ (134)		$ (186)
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	0.50%	0.20%		0.30%
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)	(134)		(186)
Prior Period Development, net Adjustments		0
Prior Year Claims and Claims Adjustment Expense		(305)
North America Agricultural Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ (104)	$ (18)		$ (61)
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	0.20%	0.00%		0.10%
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		(104)	(18)		(61)
Prior Period Development, net Adjustments		66
Prior Year Claims and Claims Adjustment Expense		(170)
Overseas General Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(290)	$ (376)		$ (448)
Prior Period Development, net Adjustments		$ 0
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	0.50%	0.60%		0.80%
Prior Year Claims and Claims Adjustment Expense		$ (290)
Overseas General Insurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(26)	$ (50)		$ (65)
Prior Period Development, net Adjustments		0
Prior Year Claims and Claims Adjustment Expense		(26)
Overseas General Insurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(264)	(326)		(383)
Prior Period Development, net Adjustments		0
Prior Year Claims and Claims Adjustment Expense		$ (264)
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		89.00%
Global Reinsurance Non-Casualty [Member] | Accident years 2018 to 2022
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss by Geographic Percentage		94.00%
Global Reinsurance Non-Casualty [Member] | Accident years 2013-2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss by Geographic Percentage		76.00%
Global Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ (25)	$ (28)		$ 22
Prior Period Development, net Adjustments		$ 4
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	0.00%	0.00%		0.00%
Prior Year Claims and Claims Adjustment Expense		$ (29)
Global Reinsurance [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		0	$ 7		$ (7)
Prior Period Development, net Adjustments		1
Prior Year Claims and Claims Adjustment Expense		(1)
Global Reinsurance [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		(25)	(35)		29
Prior Period Development, net Adjustments		3
Prior Year Claims and Claims Adjustment Expense		(28)
Corporate Segment [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ 296	$ 277		$ 359
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	0.50%	0.50%		0.60%
Corporate Segment [Member] | Long Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ 296	$ 277		$ 359
Prior Period Development, net Adjustments		0
Prior Year Claims and Claims Adjustment Expense		296
Corporate Segment [Member] | Short Tail [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Net Prior Period Development		$ 0	$ 0		$ 0

[1] Calculated based on the beginning of period consolidated net unpaid losses and loss expenses.

[2] Includes favorable development of $16 million related to long duration International A&H business.

[3] 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 $119 million, $83 million, and $232 million for 2024, 2023, and 2022, respectively.

[4] Includes premium returns associated with our Alternative Risk Solutions business, which is excluded from the triangles.