Property Casualty Loss and Loss Expense - Claims Development - Excess and Surplus Lines (Details) reported_claim in Thousands, $ in Millions	Dec. 31, 2023 USD ($) reported_claim	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	$ 3,496
Liabilities for loss and ALAE, net of reinsurance	8,535
Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	1,545
Cumulative paid losses and ALAE, net of reinsurance	707
All outstanding liabilities before, net of reinsurance	4
Liabilities for loss and ALAE, net of reinsurance	842
2014 | Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	57	$ 56	$ 58	$ 59	$ 59	$ 60	$ 64	$ 75	$ 82	$ 95
Total of incurred but not reported liabilities plus expected development on reported losses	$ 0
Cumulative number of reported claims | reported_claim	2
Cumulative paid losses and ALAE, net of reinsurance	$ 55	55	53	51	48	43	37	27	17	$ 9
2015 | Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	65	61	65	66	65	67	73	81	96
Total of incurred but not reported liabilities plus expected development on reported losses	$ 0
Cumulative number of reported claims | reported_claim	2
Cumulative paid losses and ALAE, net of reinsurance	$ 62	58	56	54	51	41	29	19	$ 8
2016 | Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	89	88	91	90	82	84	87	93
Total of incurred but not reported liabilities plus expected development on reported losses	$ 2
Cumulative number of reported claims | reported_claim	3
Cumulative paid losses and ALAE, net of reinsurance	$ 83	81	75	62	51	39	21	$ 10
2017 | Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	94	91	94	94	95	95	104
Total of incurred but not reported liabilities plus expected development on reported losses	$ 4
Cumulative number of reported claims | reported_claim	3
Cumulative paid losses and ALAE, net of reinsurance	$ 88	77	68	57	41	23	$ 11
2018 | Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	105	107	108	110	109	116
Total of incurred but not reported liabilities plus expected development on reported losses	$ 6
Cumulative number of reported claims | reported_claim	3
Cumulative paid losses and ALAE, net of reinsurance	$ 88	75	62	50	26	$ 11
2019 | Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	142	139	141	135	137
Total of incurred but not reported liabilities plus expected development on reported losses	$ 16
Cumulative number of reported claims | reported_claim	3
Cumulative paid losses and ALAE, net of reinsurance	$ 102	79	55	34	$ 13
2020 | Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	152	160	172	172
Total of incurred but not reported liabilities plus expected development on reported losses	$ 26
Cumulative number of reported claims | reported_claim	4
Cumulative paid losses and ALAE, net of reinsurance	$ 86	56	37	$ 16
2021 | Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	233	235	217
Total of incurred but not reported liabilities plus expected development on reported losses	$ 66
Cumulative number of reported claims | reported_claim	4
Cumulative paid losses and ALAE, net of reinsurance	$ 82	45	$ 17
2022 | Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	280	294
Total of incurred but not reported liabilities plus expected development on reported losses	$ 134
Cumulative number of reported claims | reported_claim	3
Cumulative paid losses and ALAE, net of reinsurance	$ 46	$ 21
2023 | Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	328
Total of incurred but not reported liabilities plus expected development on reported losses	$ 251
Cumulative number of reported claims | reported_claim	2
Cumulative paid losses and ALAE, net of reinsurance	$ 15