Reinsurance - Earned Consolidated Property Casualty Insurance Premiums on Assumed and Ceded Business (Detail) - Consolidated Property and Casualty Insurance Entity - 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]
Direct earned premiums	$ 1,195	$ 1,153	$ 3,547	$ 3,409
Assumed earned premiums	39	22	99	53
Ceded earned premiums	(43)	(42)	(123)	(119)
Earned premiums	$ 1,191	$ 1,133	$ 3,523	$ 3,343