Reinsurance - Incurred Consolidated Property Casualty Insurance Loss and Loss Expenses on Assumed and Ceded Business (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Incurred loss and loss expenses	$ 854	$ 821	$ 1,707	$ 1,545
Consolidated Property and Casualty Insurance Entity
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct incurred loss and loss expenses	770	792	1,558	1,440
Assumed incurred loss and loss expenses	20	11	35	21
Ceded incurred loss and loss expenses	4	(44)	(11)	(41)
Incurred loss and loss expenses	$ 794	$ 759	$ 1,582	$ 1,420