Property Casualty Loss And Loss Expenses (Claims Development - Personal Auto) (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 ($)
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	$ 2,588
Liabilities for loss and ALAE, net of reinsurance	5,366
Personal Auto | Personal Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	1,889
Cumulative paid losses and ALAE, net of reinsurance	1,603
All outstanding liabilities before, net of reinsurance	4
Liabilities for loss and ALAE, net of reinsurance	290
2014 | Personal Auto | Personal Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	325	$ 324	$ 325	$ 317	$ 317
Total of incurred but not reported liabilities plus expected development on reported losses	$ 0
Cumulative number of reported claims | reported_claim	105
Cumulative paid losses and ALAE, net of reinsurance	$ 320	313	298	267	$ 210
2015 | Personal Auto | Personal Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	360	356	356	343
Total of incurred but not reported liabilities plus expected development on reported losses	$ 0
Cumulative number of reported claims | reported_claim	108
Cumulative paid losses and ALAE, net of reinsurance	$ 346	325	292	$ 229
2016 | Personal Auto | Personal Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	386	384	383
Total of incurred but not reported liabilities plus expected development on reported losses	$ 3
Cumulative number of reported claims | reported_claim	110
Cumulative paid losses and ALAE, net of reinsurance	$ 351	316	$ 243
2017 | Personal Auto | Personal Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	394	412
Total of incurred but not reported liabilities plus expected development on reported losses	$ 14
Cumulative number of reported claims | reported_claim	109
Cumulative paid losses and ALAE, net of reinsurance	$ 324	$ 256
2018 | Personal Auto | Personal Insurance
Claims Development [Line Items]
Incurred losses and ALAE, net of reinsurance	424
Total of incurred but not reported liabilities plus expected development on reported losses	$ 64
Cumulative number of reported claims | reported_claim	104
Cumulative paid losses and ALAE, net of reinsurance	$ 262