Property Casualty Loss And Loss Expenses (Claims Development - Commercial Casualty) (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
Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	4,160
Cumulative paid losses and ALAE, net of reinsurance	2,421
All outstanding liabilities before, net of reinsurance	81
Liabilities for loss and ALAE, net of reinsurance	1,820
2007 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	341	$ 333	$ 338	$ 328	$ 334	$ 352	$ 379	$ 412	$ 447	$ 509
Total of incurred but not reported liabilities plus expected development on reported losses	$ 5
Cumulative number of reported claims | reported_claim	0
Cumulative paid losses and ALAE, net of reinsurance	$ 324	314	309	290	275	248	200	145	92	$ 29
2008 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	355	357	357	359	362	387	409	458	516
Total of incurred but not reported liabilities plus expected development on reported losses	$ 10
Cumulative number of reported claims | reported_claim	0
Cumulative paid losses and ALAE, net of reinsurance	$ 331	325	315	293	262	212	158	93	$ 31
2009 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	357	357	365	370	360	412	409	484
Total of incurred but not reported liabilities plus expected development on reported losses	$ 21
Cumulative number of reported claims | reported_claim	20
Cumulative paid losses and ALAE, net of reinsurance	$ 322	310	295	267	213	151	75	$ 27
2010 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	349	348	347	349	378	394	495
Total of incurred but not reported liabilities plus expected development on reported losses	$ 31
Cumulative number of reported claims | reported_claim	20
Cumulative paid losses and ALAE, net of reinsurance	$ 300	285	256	203	159	92	$ 33
2011 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	380	375	377	377	404	466
Total of incurred but not reported liabilities plus expected development on reported losses	$ 37
Cumulative number of reported claims | reported_claim	19
Cumulative paid losses and ALAE, net of reinsurance	$ 298	266	227	149	93	$ 27
2012 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	394	394	417	414	466
Total of incurred but not reported liabilities plus expected development on reported losses	$ 48
Cumulative number of reported claims | reported_claim	18
Cumulative paid losses and ALAE, net of reinsurance	$ 288	232	170	88	$ 27
2013 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	416	431	443	448
Total of incurred but not reported liabilities plus expected development on reported losses	$ 96
Cumulative number of reported claims | reported_claim	19
Cumulative paid losses and ALAE, net of reinsurance	$ 232	159	90	$ 35
2014 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	479	496	503
Total of incurred but not reported liabilities plus expected development on reported losses	$ 128
Cumulative number of reported claims | reported_claim	20
Cumulative paid losses and ALAE, net of reinsurance	$ 172	97	$ 34
2015 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	526	533
Total of incurred but not reported liabilities plus expected development on reported losses	$ 225
Cumulative number of reported claims | reported_claim	19
Cumulative paid losses and ALAE, net of reinsurance	$ 108	$ 38
2016 | Commercial Casualty Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	563
Total of incurred but not reported liabilities plus expected development on reported losses	$ 337
Cumulative number of reported claims | reported_claim	15
Cumulative paid losses and ALAE, net of reinsurance	$ 46