Reinsurance (Incurred Consolidated Property Casualty Insurance Loss And Loss Expenses On Assumed And Ceded Business) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net incurred loss and loss expenses	$ 3,390	$ 3,107	$ 2,808
Property, Liability and Casualty Insurance Product Line [Member]
Direct incurred loss and loss expenses	2,961	2,874	2,596
Assumed incurred loss and loss expenses	113	43	12
Ceded incurred loss and loss expenses	64	(56)	(36)
Net incurred loss and loss expenses	$ 3,138	$ 2,861	$ 2,572