Property Casualty Loss And Loss Expenses (Claims Development - Excess and Surplus Lines) (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 ($)	Dec. 31, 2011 USD ($)
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	$ 2,187
Liabilities for loss and ALAE, net of reinsurance	4,770
Excess and Surplus Lines Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	366
Cumulative paid losses and ALAE, net of reinsurance	144
All outstanding liabilities before, net of reinsurance	1
Liabilities for loss and ALAE, net of reinsurance	223
2011 | Excess and Surplus Lines Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	35	$ 36	$ 38	$ 44	$ 47	$ 48
Total of incurred but not reported liabilities plus expected development on reported losses	$ 0
Cumulative number of reported claims | reported_claim	1
Cumulative paid losses and ALAE, net of reinsurance	$ 32	30	27	23	14	$ 8
2012 | Excess and Surplus Lines Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	37	40	49	56	67
Total of incurred but not reported liabilities plus expected development on reported losses	$ 2
Cumulative number of reported claims | reported_claim	1
Cumulative paid losses and ALAE, net of reinsurance	$ 29	25	19	15	$ 9
2013 | Excess and Surplus Lines Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	45	54	64	74
Total of incurred but not reported liabilities plus expected development on reported losses	$ 9
Cumulative number of reported claims | reported_claim	2
Cumulative paid losses and ALAE, net of reinsurance	$ 27	20	12	$ 7
2014 | Excess and Surplus Lines Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	75	82	95
Total of incurred but not reported liabilities plus expected development on reported losses	$ 26
Cumulative number of reported claims | reported_claim	2
Cumulative paid losses and ALAE, net of reinsurance	$ 27	17	$ 9
2015 | Excess and Surplus Lines Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	81	96
Total of incurred but not reported liabilities plus expected development on reported losses	$ 40
Cumulative number of reported claims | reported_claim	2
Cumulative paid losses and ALAE, net of reinsurance	$ 19	$ 8
2016 | Excess and Surplus Lines Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	93
Total of incurred but not reported liabilities plus expected development on reported losses	$ 54
Cumulative number of reported claims | reported_claim	2
Cumulative paid losses and ALAE, net of reinsurance	$ 10