Investments (Narrative) (Detail) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($) security	Dec. 31, 2019 USD ($) security
Schedule Of Investments [Line Items]
Fair value of securities on deposit with various regulatory authorities	$ 11	$ 11
Fair value of securities pledged as collateral	$ 294	$ 313
Number of securities in unrealized loss position for a continuous 12 month period | security	45	63
Rate that a security's fair value is below book value	5.00%	5.00%
Collateral Pledged [Member]
Schedule Of Investments [Line Items]
Cash	$ 8
Securities In Unrealized Loss Position [Member]
Schedule Of Investments [Line Items]
Weighted average contractual maturity period in years for securities in an unrealized loss position	11 years	10 years
Fair Value Below Book Value Greater Than Five Percent [Member]
Schedule Of Investments [Line Items]
Number of securities in unrealized loss position for a continuous 12 month period | security	16	16