Property Casualty Loss And Loss Expenses (Claims Development - Homeowner) (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 ($)
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	$ 2,353
Liabilities for loss and ALAE, net of reinsurance	4,930
Home Owner Line | Personal Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	1,466
Cumulative paid losses and ALAE, net of reinsurance	1,358
All outstanding liabilities before, net of reinsurance	3
Liabilities for loss and ALAE, net of reinsurance	111
2013 | Home Owner Line | Personal Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	246	$ 247	$ 247	$ 246	$ 252
Total of incurred but not reported liabilities plus expected development on reported losses	$ 0
Cumulative number of reported claims | reported_claim	25
Cumulative paid losses and ALAE, net of reinsurance	$ 246	244	242	236	$ 180
2014 | Home Owner Line | Personal Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	285	286	283	297
Total of incurred but not reported liabilities plus expected development on reported losses	$ 1
Cumulative number of reported claims | reported_claim	26
Cumulative paid losses and ALAE, net of reinsurance	$ 283	281	273	$ 224
2015 | Home Owner Line | Personal Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	275	275	284
Total of incurred but not reported liabilities plus expected development on reported losses	$ 1
Cumulative number of reported claims | reported_claim	24
Cumulative paid losses and ALAE, net of reinsurance	$ 269	260	$ 203
2016 | Home Owner Line | Personal Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	304	315
Total of incurred but not reported liabilities plus expected development on reported losses	$ 3
Cumulative number of reported claims | reported_claim	23
Cumulative paid losses and ALAE, net of reinsurance	$ 283	$ 208
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Home Owner Line | Personal Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	356
Total of incurred but not reported liabilities plus expected development on reported losses	$ 8
Cumulative number of reported claims | reported_claim	24
Cumulative paid losses and ALAE, net of reinsurance	$ 277