Property Casualty Loss And Loss Expenses (Claims Development - Workers Compensation) (Details) reported_claim in Thousands, $ in Millions	Dec. 31, 2016 USD ($) reported_claim	Dec. 31, 2015 USD ($)	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 ($)	Dec. 31, 2007 USD ($)
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	$ 2,187
Liabilities for loss and ALAE, net of reinsurance	4,770
Workers' Compensation Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	2,370
Cumulative paid losses and ALAE, net of reinsurance	1,686
All outstanding liabilities before, net of reinsurance	242
Liabilities for loss and ALAE, net of reinsurance	926
2007 | Workers' Compensation Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	275	$ 277	$ 277	$ 280	$ 279	$ 277	$ 286	$ 287	$ 281	$ 286
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	$ 238	237	233	230	226	219	206	185	145	$ 67
2008 | Workers' Compensation Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	296	296	297	299	299	302	310	305	298
Total of incurred but not reported liabilities plus expected development on reported losses	$ 31
Cumulative number of reported claims | reported_claim	0
Cumulative paid losses and ALAE, net of reinsurance	$ 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	234	234	235	238	238	263	278	307
Total of incurred but not reported liabilities plus expected development on reported losses	$ 22
Cumulative number of reported claims | reported_claim	27
Cumulative paid losses and ALAE, net of reinsurance	$ 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	240	239	240	242	248	274	283
Total of incurred but not reported liabilities plus expected development on reported losses	$ 26
Cumulative number of reported claims | reported_claim	26
Cumulative paid losses and ALAE, net of reinsurance	$ 202	198	192	181	164	134	$ 67
2011 | Workers' Compensation Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	236	239	242	246	251	284
Total of incurred but not reported liabilities plus expected development on reported losses	$ 26
Cumulative number of reported claims | reported_claim	24
Cumulative paid losses and ALAE, net of reinsurance	$ 190	186	177	161	131	$ 65
2012 | Workers' Compensation Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	213	220	234	245	265
Total of incurred but not reported liabilities plus expected development on reported losses	$ 29
Cumulative number of reported claims | reported_claim	21
Cumulative paid losses and ALAE, net of reinsurance	$ 171	162	147	121	$ 62
2013 | Workers' Compensation Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	212	221	246	264
Total of incurred but not reported liabilities plus expected development on reported losses	$ 34
Cumulative number of reported claims | reported_claim	20
Cumulative paid losses and ALAE, net of reinsurance	$ 157	144	119	$ 61
2014 | Workers' Compensation Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	214	233	261
Total of incurred but not reported liabilities plus expected development on reported losses	$ 48
Cumulative number of reported claims | reported_claim	19
Cumulative paid losses and ALAE, net of reinsurance	$ 134	110	$ 56
2015 | Workers' Compensation Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	220	246
Total of incurred but not reported liabilities plus expected development on reported losses	$ 83
Cumulative number of reported claims | reported_claim	17
Cumulative paid losses and ALAE, net of reinsurance	$ 93	$ 47
2016 | Workers' Compensation Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	230
Total of incurred but not reported liabilities plus expected development on reported losses	$ 106
Cumulative number of reported claims | reported_claim	14
Cumulative paid losses and ALAE, net of reinsurance	$ 46