Unpaid losses and loss expenses (A&E Loss Roll-forward) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Liability For Asbestos And Environmental Claims Net Roll Forward
Balance (gross) at beginning of year		$ 1,510	$ 1,628	$ 1,868
Balance (net) at beginning of year		1,013	1,102	1,252
Incurred activity, gross		268	212	148
Incurred activity, net	[1]	183	132	104
Paid activity, gross		(363)	(330)	(388)
Paid activity, net		(251)	(221)	(254)
Balance (gross) at end of year		1,415	1,510	1,628
Balance (net) at end of year		945	1,013	1,102
Brandywine Run-off [Member]
Liability For Asbestos And Environmental Claims Net Roll Forward
Contributions to the dividend retention fund		75	75
Asbestos Issue [Member]
Liability For Asbestos And Environmental Claims Net Roll Forward
Balance (gross) at beginning of year		1,098	1,226	1,351
Balance (net) at beginning of year		703	800	873
Incurred activity, gross		180	87	96
Incurred activity, net		120	55	64
Paid activity, gross		(258)	(215)	(221)
Paid activity, net		(169)	(152)	(137)
Balance (gross) at end of year		1,020	1,098	1,226
Balance (net) at end of year		654	703	800
Environmental Issue [Member]
Liability For Asbestos And Environmental Claims Net Roll Forward
Balance (gross) at beginning of year		412	402	517
Balance (net) at beginning of year		310	302	379
Incurred activity, gross		88	125	52
Incurred activity, net		63	77	40
Paid activity, gross		(105)	(115)	(167)
Paid activity, net		(82)	(69)	(117)
Balance (gross) at end of year		395	412	402
Balance (net) at end of year		291	310	$ 302
Brandywine [Member]
Liability For Asbestos And Environmental Claims Net Roll Forward
Balance (net) at beginning of year		602
Balance (net) at end of year		570	602
Westchester Specialty [Member]
Liability For Asbestos And Environmental Claims Net Roll Forward
Balance (net) at beginning of year		98
Balance (net) at end of year		89	98
Other Segments [Member]
Liability For Asbestos And Environmental Claims Net Roll Forward
Balance (net) at beginning of year		47
Balance (net) at end of year		45	47
The Chubb Corporation [Member]
Liability For Asbestos And Environmental Claims Net Roll Forward
Balance (net) at beginning of year		266
Balance (net) at end of year		$ 241	$ 266

[1] Excludes unallocated loss expenses and the net activity reflects third-party reinsurance other than the aggregate excess of loss reinsurance provided by National Indemnity Company (NICO) to Westchester Specialty (see Westchester Specialty section below)