Investments - Changes in Allowance for Credit Losses by Major Security Type of Investments Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020
Schedule Of Held To Maturity Securities [Line Items]
Balance	$ 357	$ 301
Provision for credit loss benefit	(69)	(13)
Balance	288	288
States and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Balance	49	45
Provision for credit loss benefit	(11)	(7)
Balance	38	38
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Balance	299	245
Provision for credit loss benefit	(57)	(3)
Balance	242	242
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Balance	9	11
Provision for credit loss benefit	(1)	(3)
Balance	$ 8	$ 8