Claims Development - Workers Compensation (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
Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	1,865
Cumulative paid losses and ALAE, net of reinsurance	1,215
All outstanding liabilities before, net of reinsurance	311
Liabilities for loss and ALAE, net of reinsurance	961
2013 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	204	$ 204	$ 201	$ 202	$ 205	$ 208	$ 212	$ 221	$ 246	$ 264
Total of incurred but not reported liabilities plus expected development on reported losses	$ 16
Cumulative number of reported claims | reported_claim	20
Cumulative paid losses and ALAE, net of reinsurance	$ 179	177	174	170	168	164	157	144	119	$ 61
2014 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	202	202	197	198	201	203	214	233	261
Total of incurred but not reported liabilities plus expected development on reported losses	$ 16
Cumulative number of reported claims | reported_claim	19
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	171	173	173	179	195	208	220	246
Total of incurred but not reported liabilities plus expected development on reported losses	$ 23
Cumulative number of reported claims | reported_claim	17
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	183	183	183	188	206	218	230
Total of incurred but not reported liabilities plus expected development on reported losses	$ 24
Cumulative number of reported claims | reported_claim	16
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	167	172	183	190	208	218
Total of incurred but not reported liabilities plus expected development on reported losses	$ 27
Cumulative number of reported claims | reported_claim	15
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	179	186	199	207	222
Total of incurred but not reported liabilities plus expected development on reported losses	$ 28
Cumulative number of reported claims | reported_claim	15
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	188	202	215	224
Total of incurred but not reported liabilities plus expected development on reported losses	$ 41
Cumulative number of reported claims | reported_claim	14
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	172	190	204
Total of incurred but not reported liabilities plus expected development on reported losses	$ 55
Cumulative number of reported claims | reported_claim	11
Cumulative paid losses and ALAE, net of reinsurance	$ 82	68	$ 37
2021 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	190	202
Total of incurred but not reported liabilities plus expected development on reported losses	$ 49
Cumulative number of reported claims | reported_claim	11
Cumulative paid losses and ALAE, net of reinsurance	$ 82	$ 37
2022 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	209
Total of incurred but not reported liabilities plus expected development on reported losses	$ 84
Cumulative number of reported claims | reported_claim	10
Cumulative paid losses and ALAE, net of reinsurance	$ 37