Consolidated Property Casualty Loss And Loss Expense Reserves (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Feb. 28, 2019
Business Acquisition
Gross loss and loss expense reserves, January 1	$ 6,088	$ 5,646	$ 5,219
Less reinsurance recoverable	342	238	187
Net loss and loss expense reserves, January 1	5,746	5,408	5,032
Net incurred loss and loss expenses related to:
Current accident year	3,968	3,600	3,390
Prior accident years	(131)	(248)	(167)
Total incurred	3,837	3,352	3,223
Net paid loss and loss expenses related to:
Current accident year	1,493	1,462	1,391
Prior accident years	1,690	1,798	1,456
Total paid	3,183	3,260	2,847
Net loss and loss expense reserves, December 31	6,400	5,746	5,408
Plus reinsurance recoverable	277	342	238
Gross loss and loss expense reserves, December 31	$ 6,677	$ 6,088	$ 5,646
Cincinnati Global
Business Acquisition
Net loss and loss expense reserves related to acquisition of Cincinnati Global at February 28, 2019				$ 246