Reinsurance - Summary of Effects of Reinsurance on Earned Premiums (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Earned premiums, Direct	$ 11,491	$ 11,337	$ 10,936
Earned premiums, Assumed	338	355	367
Earned premiums, Ceded	4,401	4,380	4,315
Earned premiums, Net	$ 7,428	$ 7,312	$ 6,988
Percentage of amount assumed to net	4.60%	4.90%	5.30%
Property and Casualty Operations [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Earned premiums, Direct	$ 11,021	$ 10,857	$ 10,447
Earned premiums, Assumed	288	305	317
Earned premiums, Ceded	4,401	4,380	4,315
Earned premiums, Net	$ 6,908	$ 6,782	$ 6,449
Percentage of amount assumed to net	4.20%	4.50%	4.90%
Long Term Care [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Earned premiums, Direct	$ 470	$ 480	$ 489
Earned premiums, Assumed	50	50	50
Earned premiums, Ceded
Earned premiums, Net	$ 520	$ 530	$ 539
Percentage of amount assumed to net	9.60%	9.40%	9.30%