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	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2020
Schedule Of Held To Maturity Securities [Line Items]
Balance	$ 275	$ 301
Provision for credit loss expense (benefit)	82	56
Balance	357	357
States and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Balance	50	45
Provision for credit loss expense (benefit)	(1)	4
Balance	49	49
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Balance	215	245
Provision for credit loss expense (benefit)	84	54
Balance	299	299
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Balance	10	11
Provision for credit loss expense (benefit)	(1)	(2)
Balance	$ 9	$ 9