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, 2025	Dec. 31, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Net premiums earned	$ 17,065	$ 15,100	$ 12,440
Net premiums written	16,476	15,732	13,468
Operating segments \| Insurance
Segment Reporting Information [Line Items]
Net premiums earned	7,771	6,627	5,446
Net premiums written	7,798	6,874	5,862
Operating segments \| Insurance \| North America
Segment Reporting Information [Line Items]
Net premiums earned	5,735	4,702	3,781
Net premiums written	5,724	4,869	3,995
Operating segments \| Insurance \| International
Segment Reporting Information [Line Items]
Net premiums earned	2,036	1,925	1,665
Net premiums written	2,074	2,005	1,867
Operating segments \| Insurance \| Property and short-tail specialty \| North America
Segment Reporting Information [Line Items]
Net premiums earned	1,373	1,165	976
Operating segments \| Insurance \| Property and short-tail specialty \| International
Segment Reporting Information [Line Items]
Net premiums earned	1,099	1,061	885
Operating segments \| Insurance \| Casualty and other \| International
Segment Reporting Information [Line Items]
Net premiums earned	937	864	780
Operating segments \| Insurance \| Other liability - occurrence \| North America
Segment Reporting Information [Line Items]
Net premiums earned	1,321	942	618
Operating segments \| Insurance \| Other liability - claims made \| North America
Segment Reporting Information [Line Items]
Net premiums earned	786	843	866
Operating segments \| Insurance \| Commercial multi-peril \| North America
Segment Reporting Information [Line Items]
Net premiums earned	792	435	193
Operating segments \| Insurance \| Commercial automobile \| North America
Segment Reporting Information [Line Items]
Net premiums earned	581	459	343
Operating segments \| Insurance \| Workers compensation \| North America
Segment Reporting Information [Line Items]
Net premiums earned	591	549	495
Operating segments \| Insurance \| Other \| North America
Segment Reporting Information [Line Items]
Net premiums earned	291	309	290
Operating segments \| Reinsurance
Segment Reporting Information [Line Items]
Net premiums earned	8,122	7,242	5,836
Net premiums written	7,618	7,746	6,554
Operating segments \| Reinsurance \| Bermuda
Segment Reporting Information [Line Items]
Net premiums written	3,672	3,425	3,288
Operating segments \| Reinsurance \| United States
Segment Reporting Information [Line Items]
Net premiums written	1,798	2,135	1,756
Operating segments \| Reinsurance \| Europe and other
Segment Reporting Information [Line Items]
Net premiums written	2,148	2,186	1,510
Operating segments \| Reinsurance \| Other
Segment Reporting Information [Line Items]
Net premiums earned	150	152	118
Operating segments \| Reinsurance \| Specialty
Segment Reporting Information [Line Items]
Net premiums earned	2,906	2,619	2,097
Operating segments \| Reinsurance \| Property excluding property catastrophe
Segment Reporting Information [Line Items]
Net premiums earned	2,252	2,148	1,645
Operating segments \| Reinsurance \| Casualty
Segment Reporting Information [Line Items]
Net premiums earned	1,432	1,088	1,005
Operating segments \| Reinsurance \| Property catastrophe
Segment Reporting Information [Line Items]
Net premiums earned	1,065	959	742
Operating segments \| Reinsurance \| Marine and aviation
Segment Reporting Information [Line Items]
Net premiums earned	317	276	229
Operating segments \| Mortgage
Segment Reporting Information [Line Items]
Net premiums earned	1,172	1,231	1,158
Net premiums written	1,060	1,112	1,052
Operating segments \| Mortgage \| United States
Segment Reporting Information [Line Items]
Net premiums written	780	823	743
Operating segments \| Mortgage \| Other
Segment Reporting Information [Line Items]
Net premiums written	280	289	309
Operating segments \| Mortgage \| U.S. primary mortgage insurance
Segment Reporting Information [Line Items]
Net premiums earned	802	845	759
Operating segments \| Mortgage \| U.S. credit risk transfer (CRT) and other
Segment Reporting Information [Line Items]
Net premiums earned	207	213	220
Operating segments \| Mortgage \| International mortgage insurance/reinsurance
Segment Reporting Information [Line Items]
Net premiums earned	$ 163	$ 173	$ 179