Unpaid losses and loss expenses (A&E Loss Roll-forward) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Liability For Asbestos And Environmental Claims Net Roll Forward
Balance (gross) at beginning of year		$ 1,415	$ 1,510	$ 1,628
Balance (net) at beginning of year		945	1,013	1,102
Incurred activity, gross		250	268	212
Incurred activity, net	[1]	173	183	132
Paid activity, gross		(322)	(363)	(330)
Paid activity, net		(233)	(251)	(221)
Balance (gross) at end of year		1,343	1,415	1,510
Balance (net) at end of year		885	945	1,013
Brandywine Run-off [Member]
Liability For Asbestos And Environmental Claims Net Roll Forward
Contributions to the dividend retention fund		93	75
Asbestos Issue [Member]
Liability For Asbestos And Environmental Claims Net Roll Forward
Balance (gross) at beginning of year		1,020	1,098	1,226
Balance (net) at beginning of year		654	703	800
Incurred activity, gross		176	180	87
Incurred activity, net		126	120	55
Paid activity, gross		(232)	(258)	(215)
Paid activity, net		(172)	(169)	(152)
Balance (gross) at end of year		964	1,020	1,098
Balance (net) at end of year		608	654	703
Environmental Issue [Member]
Liability For Asbestos And Environmental Claims Net Roll Forward
Balance (gross) at beginning of year		395	412	402
Balance (net) at beginning of year		291	310	302
Incurred activity, gross		74	88	125
Incurred activity, net		47	63	77
Paid activity, gross		(90)	(105)	(115)
Paid activity, net		(61)	(82)	(69)
Balance (gross) at end of year		379	395	412
Balance (net) at end of year		277	291	$ 310
Brandywine [Member]
Liability For Asbestos And Environmental Claims Net Roll Forward
Balance (net) at beginning of year		570
Balance (net) at end of year		502	570
Westchester Specialty [Member]
Liability For Asbestos And Environmental Claims Net Roll Forward
Balance (net) at beginning of year		89
Balance (net) at end of year		86	89
Other Segments [Member]
Liability For Asbestos And Environmental Claims Net Roll Forward
Balance (net) at beginning of year		45
Balance (net) at end of year		39	45
The Chubb Corporation [Member]
Liability For Asbestos And Environmental Claims Net Roll Forward
Balance (net) at beginning of year		241
Balance (net) at end of year		$ 258	$ 241

[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)