Reinsurance - Summary of Effects of Reinsurance on Earned Premiums (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Earned premiums, Direct	$ 11,337	$ 10,936	$ 10,886
Earned premiums, Assumed	355	367	308
Earned premiums, Ceded	4,380	4,315	4,270
Earned premiums, Net	$ 7,312	$ 6,988	$ 6,924
Percentage of amount assumed to net	4.90%	5.30%	4.40%
Property and Casualty Operations [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Earned premiums, Direct	$ 10,857	$ 10,447	$ 10,400
Earned premiums, Assumed	305	317	258
Earned premiums, Ceded	4,380	4,315	4,270
Earned premiums, Net	$ 6,782	$ 6,449	$ 6,388
Percentage of amount assumed to net	4.50%	4.90%	4.00%
Long Term Care [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Earned premiums, Direct	$ 480	$ 489	$ 486
Earned premiums, Assumed	50	50	50
Earned premiums, Net	$ 530	$ 539	$ 536
Percentage of amount assumed to net	9.40%	9.30%	9.30%