Claims Development - Workers Compensation (Details) reported_claim in Thousands, $ in Millions	Dec. 31, 2020 USD ($) reported_claim	Dec. 31, 2019 USD ($)	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 ($)	Dec. 31, 2011 USD ($)
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	$ 3,158
Liabilities for loss and ALAE, net of reinsurance	6,394
Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	1,990
Cumulative paid losses and ALAE, net of reinsurance	1,357
All outstanding liabilities before, net of reinsurance	308
Liabilities for loss and ALAE, net of reinsurance	941
2011 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	228	$ 228	$ 229	$ 231	$ 236	$ 239	$ 242	$ 246	$ 251	$ 284
Total of incurred but not reported liabilities plus expected development on reported losses	$ 17
Cumulative number of reported claims | reported_claim	24
Cumulative paid losses and ALAE, net of reinsurance	$ 198	197	195	192	190	186	177	161	131	$ 65
2012 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	207	208	209	211	213	220	234	245	265
Total of incurred but not reported liabilities plus expected development on reported losses	$ 19
Cumulative number of reported claims | reported_claim	21
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	201	202	205	208	212	221	246	264
Total of incurred but not reported liabilities plus expected development on reported losses	$ 12
Cumulative number of reported claims | reported_claim	20
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	197	198	201	203	214	233	261
Total of incurred but not reported liabilities plus expected development on reported losses	$ 13
Cumulative number of reported claims | reported_claim	19
Cumulative paid losses and ALAE, net of reinsurance	$ 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	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	$ 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	188	206	218	230
Total of incurred but not reported liabilities plus expected development on reported losses	$ 27
Cumulative number of reported claims | reported_claim	16
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	183	190	208	218
Total of incurred but not reported liabilities plus expected development on reported losses	$ 46
Cumulative number of reported claims | reported_claim	15
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	199	207	222
Total of incurred but not reported liabilities plus expected development on reported losses	$ 52
Cumulative number of reported claims | reported_claim	15
Cumulative paid losses and ALAE, net of reinsurance	$ 115	95	$ 48
2019 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	215	224
Total of incurred but not reported liabilities plus expected development on reported losses	$ 65
Cumulative number of reported claims | reported_claim	14
Cumulative paid losses and ALAE, net of reinsurance	$ 94	$ 49
2020 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	204
Total of incurred but not reported liabilities plus expected development on reported losses	$ 100
Cumulative number of reported claims | reported_claim	10
Cumulative paid losses and ALAE, net of reinsurance	$ 37