Property Casualty Loss And Loss Expenses (Claims Development - Excess and Surplus Lines) (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 ($)
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	$ 2,353
Liabilities for loss and ALAE, net of reinsurance	4,930
Excess and Surplus Lines Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	441
Cumulative paid losses and ALAE, net of reinsurance	195
All outstanding liabilities before, net of reinsurance	94
Liabilities for loss and ALAE, net of reinsurance	340
2011 | Excess and Surplus Lines Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	35	$ 35	$ 36	$ 38	$ 44	$ 47	$ 48
Total of incurred but not reported liabilities plus expected development on reported losses	$ 1
Cumulative number of reported claims | reported_claim	1
Cumulative paid losses and ALAE, net of reinsurance	$ 34	32	30	27	23	14	$ 8
2012 | Excess and Surplus Lines Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	36	37	40	49	56	67
Total of incurred but not reported liabilities plus expected development on reported losses	$ 1
Cumulative number of reported claims | reported_claim	1
Cumulative paid losses and ALAE, net of reinsurance	$ 31	29	25	19	15	$ 9
2013 | Excess and Surplus Lines Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	42	45	54	64	74
Total of incurred but not reported liabilities plus expected development on reported losses	$ 4
Cumulative number of reported claims | reported_claim	2
Cumulative paid losses and ALAE, net of reinsurance	$ 32	27	20	12	$ 7
2014 | Excess and Surplus Lines Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	64	75	82	95
Total of incurred but not reported liabilities plus expected development on reported losses	$ 16
Cumulative number of reported claims | reported_claim	2
Cumulative paid losses and ALAE, net of reinsurance	$ 37	27	17	$ 9
2015 | Excess and Surplus Lines Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	73	81	96
Total of incurred but not reported liabilities plus expected development on reported losses	$ 22
Cumulative number of reported claims | reported_claim	2
Cumulative paid losses and ALAE, net of reinsurance	$ 29	19	$ 8
2016 | Excess and Surplus Lines Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	87	93
Total of incurred but not reported liabilities plus expected development on reported losses	$ 34
Cumulative number of reported claims | reported_claim	2
Cumulative paid losses and ALAE, net of reinsurance	$ 21	$ 10
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Excess and Surplus Lines Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	104
Cumulative paid losses and ALAE, net of reinsurance	$ 11