Property Casualty Loss And Loss Expenses (Claims Development - Workers Compensation) (Details) reported_claim in Thousands, $ in Millions	Dec. 31, 2017 USD ($) reported_claim	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	$ 2,353
Liabilities for loss and ALAE, net of reinsurance	4,930
Workers' Compensation Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	2,259
Cumulative paid losses and ALAE, net of reinsurance	1,598
All outstanding liabilities before, net of reinsurance	267
Liabilities for loss and ALAE, net of reinsurance	928
2008 | Workers' Compensation Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	293	$ 296	$ 296	$ 297	$ 299	$ 299	$ 302	$ 310	$ 305	$ 298
Total of incurred but not reported liabilities plus expected development on reported losses	$ 28
Cumulative number of reported claims | reported_claim	0
Cumulative paid losses and ALAE, net of reinsurance	$ 256	254	251	247	242	233	217	195	155	$ 72
2009 | Workers' Compensation Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	232	234	234	235	238	238	263	278	307
Total of incurred but not reported liabilities plus expected development on reported losses	$ 18
Cumulative number of reported claims | reported_claim	27
Cumulative paid losses and ALAE, net of reinsurance	$ 202	201	198	193	188	179	163	132	$ 65
2010 | Workers' Compensation Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	237	240	239	240	242	248	274	283
Total of incurred but not reported liabilities plus expected development on reported losses	$ 22
Cumulative number of reported claims | reported_claim	26
Cumulative paid losses and ALAE, net of reinsurance	$ 204	202	198	192	181	164	134	$ 67
2011 | Workers' Compensation Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	231	236	239	242	246	251	284
Total of incurred but not reported liabilities plus expected development on reported losses	$ 22
Cumulative number of reported claims | reported_claim	24
Cumulative paid losses and ALAE, net of reinsurance	$ 192	190	186	177	161	131	$ 65
2012 | Workers' Compensation Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	211	213	220	234	245	265
Total of incurred but not reported liabilities plus expected development on reported losses	$ 26
Cumulative number of reported claims | reported_claim	21
Cumulative paid losses and ALAE, net of reinsurance	$ 175	171	162	147	121	$ 62
2013 | Workers' Compensation Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	208	212	221	246	264
Total of incurred but not reported liabilities plus expected development on reported losses	$ 28
Cumulative number of reported claims | reported_claim	20
Cumulative paid losses and ALAE, net of reinsurance	$ 164	157	144	119	$ 61
2014 | Workers' Compensation Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	203	214	233	261
Total of incurred but not reported liabilities plus expected development on reported losses	$ 31
Cumulative number of reported claims | reported_claim	19
Cumulative paid losses and ALAE, net of reinsurance	$ 148	134	110	$ 56
2015 | Workers' Compensation Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	208	220	246
Total of incurred but not reported liabilities plus expected development on reported losses	$ 60
Cumulative number of reported claims | reported_claim	17
Cumulative paid losses and ALAE, net of reinsurance	$ 115	93	$ 47
2016 | Workers' Compensation Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	218	230
Total of incurred but not reported liabilities plus expected development on reported losses	$ 72
Cumulative number of reported claims | reported_claim	16
Cumulative paid losses and ALAE, net of reinsurance	$ 97	$ 46
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Workers' Compensation Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	218
Total of incurred but not reported liabilities plus expected development on reported losses	$ 89
Cumulative number of reported claims | reported_claim	13
Cumulative paid losses and ALAE, net of reinsurance	$ 45