Claims Development - Workers Compensation (Details) reported_claim in Thousands, $ in Millions	Dec. 31, 2021 USD ($) reported_claim	Dec. 31, 2020 USD ($)	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 ($)	Dec. 31, 2012 USD ($)
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	$ 3,118
Liabilities for loss and ALAE, net of reinsurance	6,917
Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	1,923
Cumulative paid losses and ALAE, net of reinsurance	1,279
All outstanding liabilities before, net of reinsurance	301
Liabilities for loss and ALAE, net of reinsurance	945
2012 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	209	$ 207	$ 208	$ 209	$ 211	$ 213	$ 220	$ 234	$ 245	$ 265
Total of incurred but not reported liabilities plus expected development on reported losses	$ 20
Cumulative number of reported claims | reported_claim	21
Cumulative paid losses and ALAE, net of reinsurance	$ 183	182	180	178	175	171	162	147	121	$ 62
2013 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	204	201	202	205	208	212	221	246	264
Total of incurred but not reported liabilities plus expected development on reported losses	$ 17
Cumulative number of reported claims | reported_claim	20
Cumulative paid losses and ALAE, net of reinsurance	$ 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	197	198	201	203	214	233	261
Total of incurred but not reported liabilities plus expected development on reported losses	$ 17
Cumulative number of reported claims | reported_claim	19
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	173	173	179	195	208	220	246
Total of incurred but not reported liabilities plus expected development on reported losses	$ 26
Cumulative number of reported claims | reported_claim	17
Cumulative paid losses and ALAE, net of reinsurance	$ 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	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	$ 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,	172	183	190	208	218
Total of incurred but not reported liabilities plus expected development on reported losses	$ 31
Cumulative number of reported claims | reported_claim	15
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	186	199	207	222
Total of incurred but not reported liabilities plus expected development on reported losses	$ 34
Cumulative number of reported claims | reported_claim	15
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	202	215	224
Total of incurred but not reported liabilities plus expected development on reported losses	$ 50
Cumulative number of reported claims | reported_claim	14
Cumulative paid losses and ALAE, net of reinsurance	$ 115	94	$ 49
2020 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	190	204
Total of incurred but not reported liabilities plus expected development on reported losses	$ 68
Cumulative number of reported claims | reported_claim	11
Cumulative paid losses and ALAE, net of reinsurance	$ 68	$ 37
2021 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	202
Total of incurred but not reported liabilities plus expected development on reported losses	$ 72
Cumulative number of reported claims | reported_claim	10
Cumulative paid losses and ALAE, net of reinsurance	$ 37