Segment Information - Summary of net premiums written and earned by major line of business and net premiums written by location (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Net premiums earned	$ 15,100	$ 12,440	$ 9,679
Net premiums written	15,732	13,468	11,078
Operating segments | Insurance
Segment Reporting Information [Line Items]
Net premiums earned	6,627	5,446	4,560
Net premiums written	6,874	5,862	5,021
Operating segments | Insurance | North America
Segment Reporting Information [Line Items]
Net premiums earned	4,702	3,781	3,283
Net premiums written	4,869	3,995	3,527
Operating segments | Insurance | International
Segment Reporting Information [Line Items]
Net premiums earned	1,925	1,665	1,277
Net premiums written	2,005	1,867	1,494
Operating segments | Insurance | Property and short-tail specialty | North America
Segment Reporting Information [Line Items]
Net premiums earned	1,165	976	746
Operating segments | Insurance | Property and short-tail specialty | International
Segment Reporting Information [Line Items]
Net premiums earned	1,050	885	613
Operating segments | Insurance | Casualty and other | International
Segment Reporting Information [Line Items]
Net premiums earned	875	780	664
Operating segments | Insurance | Other liability - occurrence | North America
Segment Reporting Information [Line Items]
Net premiums earned	942	618	509
Operating segments | Insurance | Other liability - claims made | North America
Segment Reporting Information [Line Items]
Net premiums earned	843	866	874
Operating segments | Insurance | Workers compensation | North America
Segment Reporting Information [Line Items]
Net premiums earned	549	495	415
Operating segments | Insurance | Commercial automobile | North America
Segment Reporting Information [Line Items]
Net premiums earned	459	343	287
Operating segments | Insurance | Commercial multi-peril | North America
Segment Reporting Information [Line Items]
Net premiums earned	435	193	197
Operating segments | Insurance | Other | North America
Segment Reporting Information [Line Items]
Net premiums earned	309	290	255
Operating segments | Reinsurance
Segment Reporting Information [Line Items]
Net premiums earned	7,242	5,836	3,959
Net premiums written	7,746	6,554	4,924
Operating segments | Reinsurance | Bermuda
Segment Reporting Information [Line Items]
Net premiums written	3,425	3,288	2,561
Operating segments | Reinsurance | United States
Segment Reporting Information [Line Items]
Net premiums written	2,135	1,756	1,247
Operating segments | Reinsurance | Europe and other
Segment Reporting Information [Line Items]
Net premiums written	2,186	1,510	1,116
Operating segments | Reinsurance | Other
Segment Reporting Information [Line Items]
Net premiums earned	152	118	110
Operating segments | Reinsurance | Other specialty
Segment Reporting Information [Line Items]
Net premiums earned	2,619	2,097	1,378
Operating segments | Reinsurance | Property excluding property catastrophe
Segment Reporting Information [Line Items]
Net premiums earned	2,148	1,645	1,090
Operating segments | Reinsurance | Casualty
Segment Reporting Information [Line Items]
Net premiums earned	1,088	1,005	855
Operating segments | Reinsurance | Property catastrophe
Segment Reporting Information [Line Items]
Net premiums earned	959	742	367
Operating segments | Reinsurance | Marine and aviation
Segment Reporting Information [Line Items]
Net premiums earned	276	229	159
Operating segments | Mortgage
Segment Reporting Information [Line Items]
Net premiums earned	1,231	1,158	1,160
Net premiums written	1,112	1,052	1,133
Operating segments | Mortgage | United States
Segment Reporting Information [Line Items]
Net premiums written	823	743	781
Operating segments | Mortgage | Other
Segment Reporting Information [Line Items]
Net premiums written	289	309	352
Operating segments | Mortgage | U.S. primary mortgage insurance
Segment Reporting Information [Line Items]
Net premiums earned	845	759	804
Operating segments | Mortgage | U.S. credit risk transfer (CRT) and other
Segment Reporting Information [Line Items]
Net premiums earned	213	220	196
Operating segments | Mortgage | International mortgage insurance/reinsurance
Segment Reporting Information [Line Items]
Net premiums earned	$ 173	$ 179	$ 160