Reinsurance - Summary of Effects of Reinsurance on Earned Premiums (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Earned premiums, Direct	$ 10,886	$ 10,351	$ 9,960
Earned premiums, Assumed	308	324	325
Earned premiums, Ceded	4,270	3,754	3,073
Earned premiums, Net	$ 6,924	$ 6,921	$ 7,212
Percentage of amount assumed to net	4.40%	4.70%	4.50%
Property and Casualty Operations [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Earned premiums, Direct	$ 10,400	$ 9,853	$ 9,452
Earned premiums, Assumed	258	274	277
Earned premiums, Ceded	4,270	3,754	3,073
Earned premiums, Net	$ 6,388	$ 6,373	$ 6,656
Percentage of amount assumed to net	4.00%	4.30%	4.20%
Long Term Care [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Earned premiums, Direct	$ 486	$ 498	$ 508
Earned premiums, Assumed	50	50	48
Earned premiums, Net	$ 536	$ 548	$ 556
Percentage of amount assumed to net	9.30%	9.10%	8.60%