Schedule II. Condensed Financial Information of Registrant - Condensed Statements of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating activities
Net cash provided by operating activities	$ 63,398	$ 78,824	$ 98,268
Investing activities
Purchases of property and equipment	(921)	(1,018)	(1,126)
Net cash provided by (used in) investing activities	43,371	11,688	(29,059)
Financing activities
Proceeds from stock option exercises		20	67
Finance lease purchases	(50)	(47)
Dividends to shareholders	(88,775)	(87,016)	(84,491)
Net cash used in financing activities	(88,825)	(87,043)	(84,424)
Change in cash and cash equivalents	17,944	3,469	(15,215)
Cash and cash equivalents at beginning of year	43,813	40,344	55,559
Cash and cash equivalents at end of year	61,757	43,813	40,344
Parent Company [Member]
Operating activities
Net cash provided by operating activities	6,012	2,333	2,448
Investing activities
Purchases of investments	(10,982)	(27,140)	(47,825)
Proceeds from sales of investments	18,970	1,987	54,600
Purchases of property and equipment	(764)	(995)	(1,041)
Dividends from subsidiary	88,600	115,900	65,400
Net cash provided by (used in) investing activities	95,824	89,752	71,134
Financing activities
Proceeds from stock option exercises		20	67
Finance lease purchases	(50)	(47)
Dividends to shareholders	(88,775)	(87,016)	(84,491)
Net cash used in financing activities	(88,825)	(87,043)	(84,424)
Change in cash and cash equivalents	13,011	5,042	(10,842)
Cash and cash equivalents at beginning of year	10,551	5,509	16,351
Cash and cash equivalents at end of year	$ 23,562	$ 10,551	$ 5,509