Reinsurance - Incurred Consolidated Property Casualty Insurance Loss and Loss Expenses on Assumed and Ceded Business (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Incurred loss and loss expenses	$ 874	$ 753	$ 2,581	$ 2,298
Consolidated Property and Casualty Insurance Entity
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct incurred loss and loss expenses	760	691	2,318	2,131
Assumed incurred loss and loss expenses	62	9	97	30
Ceded incurred loss and loss expenses	(7)	(10)	(18)	(51)
Incurred loss and loss expenses	$ 815	$ 690	$ 2,397	$ 2,110