Consolidated Property Casualty Loss And Loss Expense Reserves (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Feb. 28, 2019
Business Acquisition
Gross loss and loss expense reserves, January 1	$ 5,646	$ 5,219	$ 5,035
Less reinsurance recoverable	238	187	298
Net loss and loss expense reserves, January 1	5,408	5,032	4,737
Net incurred loss and loss expenses related to:
Current accident year	3,600	3,390	3,257
Prior accident years	(248)	(167)	(119)
Total incurred	3,352	3,223	3,138
Net paid loss and loss expenses related to:
Current accident year	1,462	1,391	1,404
Prior accident years	1,798	1,456	1,439
Total paid	3,260	2,847	2,843
Net loss and loss expense reserves, December 31	5,746	5,408	5,032
Plus reinsurance recoverable	342	238	187
Gross loss and loss expense reserves, December 31	$ 6,088	$ 5,646	$ 5,219
Cincinnati Global
Business Acquisition
Net loss and loss expense reserves related to acquisition of Cincinnati Global at February 28, 2019				$ 246