Claims Development - Workers Compensation (Details) reported_claim in Thousands, $ in Millions	Dec. 31, 2019 USD ($) reported_claim	Dec. 31, 2018 USD ($)	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 ($)	Dec. 31, 2010 USD ($)
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	$ 2,739
Liabilities for loss and ALAE, net of reinsurance	5,799
Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	2,061
Cumulative paid losses and ALAE, net of reinsurance	1,433
All outstanding liabilities before, net of reinsurance	295
Liabilities for loss and ALAE, net of reinsurance	923
2010 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	237	$ 236	$ 237	$ 240	$ 239	$ 240	$ 242	$ 248	$ 274	$ 283
Total of incurred but not reported liabilities plus expected development on reported losses	$ 19
Cumulative number of reported claims | reported_claim	26
Cumulative paid losses and ALAE, net of reinsurance	$ 209	207	204	202	198	192	181	164	134	$ 67
2011 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	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	$ 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,	208	209	211	213	220	234	245	265
Total of incurred but not reported liabilities plus expected development on reported losses	$ 21
Cumulative number of reported claims | reported_claim	21
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	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	$ 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,	198	201	203	214	233	261
Total of incurred but not reported liabilities plus expected development on reported losses	$ 18
Cumulative number of reported claims | reported_claim	19
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	179	195	208	220	246
Total of incurred but not reported liabilities plus expected development on reported losses	$ 33
Cumulative number of reported claims | reported_claim	17
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	188	206	218	230
Total of incurred but not reported liabilities plus expected development on reported losses	$ 31
Cumulative number of reported claims | reported_claim	16
Cumulative paid losses and ALAE, net of reinsurance	$ 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,	190	208	218
Total of incurred but not reported liabilities plus expected development on reported losses	$ 56
Cumulative number of reported claims | reported_claim	15
Cumulative paid losses and ALAE, net of reinsurance	$ 106	88	$ 45
2018 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	207	222
Total of incurred but not reported liabilities plus expected development on reported losses	$ 65
Cumulative number of reported claims | reported_claim	15
Cumulative paid losses and ALAE, net of reinsurance	$ 95	$ 48
Short-Duration Insurance Contract, Accident Year 2019 | Workers' Compensation Line | Commercial Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	224
Total of incurred but not reported liabilities plus expected development on reported losses	$ 90
Cumulative number of reported claims | reported_claim	13
Cumulative paid losses and ALAE, net of reinsurance	$ 49