Summary of Net Realized Investment Gains and Losses (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net Realized Investment Gains and Losses
Gross realized gains	$ 12	$ 52	$ 49
Gross realized losses	10	11	89
Non-credit losses included in other comprehensive income	$ 3	$ 4	$ 6

This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

No definition available.

Name:	us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration