Property Casualty Loss And Loss Expenses (Consolidated Property Casualty Loss And Loss Expense Reserves) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Insurance [Abstract]
Gross loss and loss expense reserves, January 1	$ 5,035	$ 4,660	$ 4,438
Less reinsurance recoverable	298	281	282
Net loss and loss expense reserves, January 1	4,737	4,379	4,156
Net incurred loss and loss expenses related to:
Current accident year	3,257	3,029	2,756
Prior accident years	(119)	(168)	(184)
Total incurred	3,138	2,861	2,572
Net paid loss and loss expenses related to:
Current accident year	1,404	1,260	1,152
Prior accident years	1,439	1,243	1,197
Total paid	2,843	2,503	2,349
Net loss and loss expense reserves, December 31	5,032	4,737	4,379
Plus reinsurance recoverable	187	298	281
Gross loss and loss expense reserves, December 31	$ 5,219	$ 5,035	$ 4,660