Property Casualty Loss and Loss Expense - Claims Development - Workers Compensation (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
Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	1,826
Cumulative paid losses and ALAE, net of reinsurance	1,160
All outstanding liabilities before, net of reinsurance	316
Liabilities for loss and ALAE, net of reinsurance	982
2014 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	203	$ 202	$ 202	$ 197	$ 198	$ 201	$ 203	$ 214	$ 233	$ 261
Total of incurred but not reported liabilities plus expected development on reported losses	$ 15
Cumulative number of reported claims | reported_claim	19
Cumulative paid losses and ALAE, net of reinsurance	$ 172	171	168	165	162	157	148	134	110	$ 56
2015 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	167	171	173	173	179	195	208	220	246
Total of incurred but not reported liabilities plus expected development on reported losses	$ 20
Cumulative number of reported claims | reported_claim	17
Cumulative paid losses and ALAE, net of reinsurance	$ 140	139	139	137	134	129	115	93	$ 47
2016 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	181	183	183	183	188	206	218	230
Total of incurred but not reported liabilities plus expected development on reported losses	$ 22
Cumulative number of reported claims | reported_claim	16
Cumulative paid losses and ALAE, net of reinsurance	$ 150	148	146	141	131	119	97	$ 46
2017 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	160	167	172	183	190	208	218
Total of incurred but not reported liabilities plus expected development on reported losses	$ 21
Cumulative number of reported claims | reported_claim	15
Cumulative paid losses and ALAE, net of reinsurance	$ 126	122	119	114	106	88	$ 45
2018 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	175	179	186	199	207	222
Total of incurred but not reported liabilities plus expected development on reported losses	$ 26
Cumulative number of reported claims | reported_claim	15
Cumulative paid losses and ALAE, net of reinsurance	$ 135	133	127	115	95	$ 48
2019 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	178	188	202	215	224
Total of incurred but not reported liabilities plus expected development on reported losses	$ 31
Cumulative number of reported claims | reported_claim	14
Cumulative paid losses and ALAE, net of reinsurance	$ 129	122	115	94	$ 49
2020 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	153	172	190	204
Total of incurred but not reported liabilities plus expected development on reported losses	$ 39
Cumulative number of reported claims | reported_claim	11
Cumulative paid losses and ALAE, net of reinsurance	$ 96	82	68	$ 37
2021 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	183	190	202
Total of incurred but not reported liabilities plus expected development on reported losses	$ 42
Cumulative number of reported claims | reported_claim	11
Cumulative paid losses and ALAE, net of reinsurance	$ 100	82	$ 37
2022 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	205	209
Total of incurred but not reported liabilities plus expected development on reported losses	$ 65
Cumulative number of reported claims | reported_claim	11
Cumulative paid losses and ALAE, net of reinsurance	$ 76	$ 37
2023 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	221
Total of incurred but not reported liabilities plus expected development on reported losses	$ 106
Cumulative number of reported claims | reported_claim	9
Cumulative paid losses and ALAE, net of reinsurance	$ 36