Investments - Changes in Allowance for Credit Losses by Major Security Type of Investments Classified as Held-to-Maturity (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($)
Schedule Of Held To Maturity Securities [Line Items]
Balance at January 1, 2020	$ 301
Provision for credit loss expense (benefit)	(26)
Balance at March 31, 2020	275
States and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Balance at January 1, 2020	45
Provision for credit loss expense (benefit)	5
Balance at March 31, 2020	50
Corporate Bonds [Member]
Schedule Of Held To Maturity Securities [Line Items]
Balance at January 1, 2020	245
Provision for credit loss expense (benefit)	(30)
Balance at March 31, 2020	215
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Balance at January 1, 2020	11
Provision for credit loss expense (benefit)	(1)
Balance at March 31, 2020	$ 10