Schedule II - Condensed Financial Information of Registrant - Condensed Statements of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities:
Net cash provided by (used in) operating activities	$ 234,378	$ 29,348	$ 84,598
Cash flows from investing activities:
Capital contributions to affiliates	(92,000)	(114)	0
Purchases of equity securities	(55,447)	(116,265)	(1,351)
Purchases of available-for-sale debt securities	(450,383)	(1,074,629)	(221,647)
Proceeds from sales of equity securities	85,103	81,559	29,680
Proceeds from sales of available-for-sale debt securities	96,966	1,008,436	77,790
Net cash provided by (used in) investing activities	(229,404)	22,042	28,326
Cash flows from financing activities:
Proceeds from issuance of long-term debt	100,000	0	0
Debt issuance costs paid	3,365
Preferred stock dividend	(10)	(10)	(10)
Common stock dividend	(24,191)	(24,547)	(26,106)
Cash received for strike price and tax withholdings	0	0	239
Purchase of treasury stock	(1,609)	(28,921)	(66,186)
Payments related to tax withholding for share-based compensation	(1,056)	(1,315)	(3,709)
Net cash provided by (used in) financing activities	68,298	(56,263)	(97,243)
Net increase (decrease) during the period	73,272	(4,873)	15,681
Balance, beginning of period	179,871	184,744	169,063
Balance, end of period	253,143	179,871	184,744
Parent Company
Cash flows from operating activities:
Net cash provided by (used in) operating activities	151,952	149,329	84,752
Cash flows from investing activities:
Capital contributions to affiliates	(95,498)	(118,897)	0
Issuance of note receivable to subsidiary	(20,000)	0	0
Purchases of equity securities	0	0	(107)
Purchases of available-for-sale debt securities	0	0	(3,750)
Proceeds from sales of equity securities	0	0	3,481
Proceeds from sales of available-for-sale debt securities	0	787	6,530
Net cash provided by (used in) investing activities	(115,498)	(118,110)	6,154
Cash flows from financing activities:
Proceeds from issuance of long-term debt	100,000	0	0
Debt issuance costs paid	(3,365)	0	0
Preferred stock dividend	(10)	(10)	(10)
Common stock dividend	(24,191)	(24,547)	(26,106)
Cash received for strike price and tax withholdings	0	0	239
Purchase of treasury stock	(1,609)	(28,921)	(66,186)
Payments related to tax withholding for share-based compensation	(1,056)	(1,315)	(3,709)
Net cash provided by (used in) financing activities	69,769	(54,793)	(95,772)
Net increase (decrease) during the period	106,223	(23,574)	(4,866)
Balance, beginning of period	62,934	86,508	91,374
Balance, end of period	$ 169,157	$ 62,934	$ 86,508