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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Incurred loss and loss expenses	$ 753	$ 670	$ 2,298	$ 2,131
Consolidated Property and Casualty Insurance Entity
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct incurred loss and loss expenses	691	618	2,131	1,984
Assumed incurred loss and loss expenses	9	2	30	2
Ceded incurred loss and loss expenses	(10)	(7)	(51)	(30)
Incurred loss and loss expenses	$ 690	$ 613	$ 2,110	$ 1,956