Investments - A Summary of the Company's Realized Gains and Losses on Sales, Calls or Redemptions of Investments (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
Marketable Securities [Line Items]
Proceeds from sales									$ 72,395	$ 55,090	$ 15,028
Gross realized investment gains									1,017	176	239
Gross realized investment (losses)									(18)	(68)	(1,354)
Net realized investment gains (losses)									999	108	(1,115)
Other, including gains (losses) on calls and redemptions									133	(188)	(421)
Net realized gains (losses) on investments	$ (332)	$ 309	$ 163	$ 992	$ (53)	$ (4)	$ (82)	$ 59	1,132	(80)	(1,536)
Fixed Maturity Securities Available for Sale [Member]
Marketable Securities [Line Items]
Proceeds from sales									25,436	33,113	15,025
Gross realized investment gains									711	140	238
Gross realized investment (losses)									(14)	(68)	(1,354)
Net realized investment gains (losses)									697	72	(1,116)
Other, including gains (losses) on calls and redemptions									(43)	(196)	(144)
Net realized gains (losses) on investments									654	(124)	(1,260)
Equity Securities [Member]
Marketable Securities [Line Items]
Proceeds from sales											3
Gross realized investment gains											1
Net realized investment gains (losses)											1
Net realized gains (losses) on investments											1
Other [Member]
Marketable Securities [Line Items]
Proceeds from sales									46,959	21,977
Gross realized investment gains									306	36
Gross realized investment (losses)									(4)
Net realized investment gains (losses)									302	36
Other, including gains (losses) on calls and redemptions									176	8	(277)
Net realized gains (losses) on investments									$ 478	$ 44	$ (277)