Property Casualty Loss And Loss Expenses (Claims Development - Excess and Surplus Lines) (Details) reported_claim in Thousands, $ in Millions	Dec. 31, 2018 USD ($) reported_claim	Dec. 31, 2017 USD ($)	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	$ 2,588
Liabilities for loss and ALAE, net of reinsurance	5,366
Excess and Surplus Lines Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	532
Cumulative paid losses and ALAE, net of reinsurance	256
All outstanding liabilities before, net of reinsurance	1
Liabilities for loss and ALAE, net of reinsurance	277
2011 | Excess and Surplus Lines Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	34	$ 35	$ 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	$ 33	34	32	30	27	23	14	$ 8
2012 | Excess and Surplus Lines Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	35	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	$ 32	31	29	25	19	15	$ 9
2013 | Excess and Surplus Lines Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	41	42	45	54	64	74
Total of incurred but not reported liabilities plus expected development on reported losses	$ 3
Cumulative number of reported claims | reported_claim	2
Cumulative paid losses and ALAE, net of reinsurance	$ 34	32	27	20	12	$ 7
2014 | Excess and Surplus Lines Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	60	64	75	82	95
Total of incurred but not reported liabilities plus expected development on reported losses	$ 10
Cumulative number of reported claims | reported_claim	2
Cumulative paid losses and ALAE, net of reinsurance	$ 43	37	27	17	$ 9
2015 | Excess and Surplus Lines Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	67	73	81	96
Total of incurred but not reported liabilities plus expected development on reported losses	$ 13
Cumulative number of reported claims | reported_claim	2
Cumulative paid losses and ALAE, net of reinsurance	$ 41	29	19	$ 8
2016 | Excess and Surplus Lines Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	84	87	93
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	$ 39	21	$ 10
2017 | Excess and Surplus Lines Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	95	104
Total of incurred but not reported liabilities plus expected development on reported losses	$ 43
Cumulative number of reported claims | reported_claim	2
Cumulative paid losses and ALAE, net of reinsurance	$ 23	$ 11
2018 | Excess and Surplus Lines Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	116
Total of incurred but not reported liabilities plus expected development on reported losses	$ 71
Cumulative number of reported claims | reported_claim	2
Cumulative paid losses and ALAE, net of reinsurance	$ 11