Investments - Major Categories of the Company's Net Investment Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Investment Income Reported Amounts By Category [Line Items]
Total gross investment income									$ 33,449	$ 31,418	$ 30,383
Investment expenses									(966)	(966)	(1,102)
Net investment income	$ 8,035	$ 8,264	$ 8,169	$ 8,015	$ 8,056	$ 7,884	$ 7,303	$ 7,209	32,483	30,452	29,281
Fixed Maturity Securities Available for Sale [Member]
Schedule Of Investment Income Reported Amounts By Category [Line Items]
Total gross investment income									30,343	28,762	28,961
Equity Securities [Member]
Schedule Of Investment Income Reported Amounts By Category [Line Items]
Total gross investment income									628	448	201
Short-term Investments [Member]
Schedule Of Investment Income Reported Amounts By Category [Line Items]
Total gross investment income									$ 2,478	$ 2,208	1,117
Other Investments [Member]
Schedule Of Investment Income Reported Amounts By Category [Line Items]
Total gross investment income											$ 104