Schedule II. Condensed Financial Information of Registrant - Condensed Statements of 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
Revenues
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
Net unrealized gains (losses) on equity securities	6,553	844	5,570	(8,763)	1,593	365	642	2,158	4,204	4,758	(2,088)
Fee and other income									350	321	599
Total revenues	88,221	83,005	89,082	79,169	91,939	91,488	91,753	95,190	339,477	370,370	377,753
Expenses
Total expenses									232,558	254,853	290,172
Income tax expense (benefit)									20,317	22,827	15,949
Net income	$ 28,501	$ 23,353	$ 23,948	$ 10,800	$ 34,014	$ 21,386	$ 17,890	$ 19,400	86,602	92,690	71,632
Parent Company [Member]
Revenues
Net investment income									944	915	1,089
Net unrealized gains (losses) on equity securities									(276)	1,977	(948)
Fee and other income									9,122	5,352	7,389
Total revenues									9,790	8,244	7,530
Expenses
Other operating costs									9,122	8,244	7,530
Total expenses									9,122	8,244	7,530
Income before income taxes and equity in earnings of subsidiaries									668
Income tax expense (benefit)									259	376	(36)
Gain (loss) before equity in earnings of subsidiaries									409	(376)	36
Equity in net income of subsidiaries									86,193	93,066	71,596
Net income									$ 86,602	$ 92,690	$ 71,632