Investments - Major Categories of the Company's Net Investment Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Investment Income Reported Amounts By Category [Line Items]
Total gross investment income									$ 30,317	$ 33,449	$ 31,418
Investment expenses									(953)	(966)	(966)
Net investment income	$ 7,228	$ 7,063	$ 7,324	$ 7,749	$ 8,035	$ 8,264	$ 8,169	$ 8,015	29,364	32,483	30,452
Fixed Maturity Securities Available for Sale [Member]
Schedule Of Investment Income Reported Amounts By Category [Line Items]
Total gross investment income									28,414	30,343	28,762
Equity Securities [Member]
Schedule Of Investment Income Reported Amounts By Category [Line Items]
Total gross investment income									1,006	628	448
Short-term Investments [Member]
Schedule Of Investment Income Reported Amounts By Category [Line Items]
Total gross investment income									$ 897	$ 2,478	$ 2,208