Property Casualty Loss And Loss Expenses (Claims Development - Commercial Property) (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 ($)
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	$ 2,187
Liabilities for loss and ALAE, net of reinsurance	4,770
Commercial Property Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	2,187
Cumulative paid losses and ALAE, net of reinsurance	1,910
All outstanding liabilities before, net of reinsurance	11
Liabilities for loss and ALAE, net of reinsurance	288
2012 | Commercial Property Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	386	$ 383	$ 388	$ 389	$ 403
Total of incurred but not reported liabilities plus expected development on reported losses	$ 0
Cumulative number of reported claims | reported_claim	16
Cumulative paid losses and ALAE, net of reinsurance	$ 383	380	376	356	$ 255
2013 | Commercial Property Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	356	352	359	389
Total of incurred but not reported liabilities plus expected development on reported losses	$ 2
Cumulative number of reported claims | reported_claim	14
Cumulative paid losses and ALAE, net of reinsurance	$ 349	344	330	$ 227
2014 | Commercial Property Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	441	444	452
Total of incurred but not reported liabilities plus expected development on reported losses	$ 0
Cumulative number of reported claims | reported_claim	17
Cumulative paid losses and ALAE, net of reinsurance	$ 432	412	$ 297
2015 | Commercial Property Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	414	454
Total of incurred but not reported liabilities plus expected development on reported losses	$ 5
Cumulative number of reported claims | reported_claim	16
Cumulative paid losses and ALAE, net of reinsurance	$ 388	$ 279
2016 | Commercial Property Line | Commercial Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	590
Total of incurred but not reported liabilities plus expected development on reported losses	$ 38
Cumulative number of reported claims | reported_claim	16
Cumulative paid losses and ALAE, net of reinsurance	$ 358