Claims Development - Excess and Surplus Lines (Details) reported_claim in Thousands, $ in Millions	Dec. 31, 2022 USD ($) reported_claim	Dec. 31, 2021 USD ($)	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 ($)	Dec. 31, 2013 USD ($)
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	$ 3,312
Liabilities for loss and ALAE, net of reinsurance	7,967
Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	1,272
Cumulative paid losses and ALAE, net of reinsurance	587
All outstanding liabilities before, net of reinsurance	2
Liabilities for loss and ALAE, net of reinsurance	687
2013 | Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	41	$ 40	$ 41	$ 41	$ 41	$ 42	$ 45	$ 54	$ 64	$ 74
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	$ 40	39	39	37	34	32	27	20	12	$ 7
2014 | Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	56	58	59	59	60	64	75	82	95
Total of incurred but not reported liabilities plus expected development on reported losses	$ 1
Cumulative number of reported claims | reported_claim	2
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	61	65	66	65	67	73	81	96
Total of incurred but not reported liabilities plus expected development on reported losses	$ 1
Cumulative number of reported claims | reported_claim	2
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	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	$ 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,	91	94	94	95	95	104
Total of incurred but not reported liabilities plus expected development on reported losses	$ 5
Cumulative number of reported claims | reported_claim	3
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	107	108	110	109	116
Total of incurred but not reported liabilities plus expected development on reported losses	$ 14
Cumulative number of reported claims | reported_claim	3
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	139	141	135	137
Total of incurred but not reported liabilities plus expected development on reported losses	$ 28
Cumulative number of reported claims | reported_claim	3
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	160	172	172
Total of incurred but not reported liabilities plus expected development on reported losses	$ 51
Cumulative number of reported claims | reported_claim	3
Cumulative paid losses and ALAE, net of reinsurance	$ 56	37	$ 16
2021 | Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	235	217
Total of incurred but not reported liabilities plus expected development on reported losses	$ 98
Cumulative number of reported claims | reported_claim	3
Cumulative paid losses and ALAE, net of reinsurance	$ 45	$ 17
2022 | Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	294
Total of incurred but not reported liabilities plus expected development on reported losses	$ 198
Cumulative number of reported claims | reported_claim	2
Cumulative paid losses and ALAE, net of reinsurance	$ 21