Schedule II - Condensed Financial Information of Registrant (Parent Company) - Condensed Statements of Earnings and Comprehensive Earnings (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Condensed Statement Of Income Captions [Line Items]
Net investment income	$ 68,862	$ 67,893	$ 68,870
Net realized gains	64,222	17,885	17,520
Equity in earnings of unconsolidated investees	37,060	20,233	20,960
Interest on debt	(7,677)	(7,603)	(7,588)
Earnings before income taxes	344,321	189,841	232,734
Income tax expense	64,967	32,750	41,092
Net earnings	279,354	157,091	191,642
Unrealized gains (losses) on securities:
Net current period other comprehensive earnings (loss)	(58,888)	56,219	67,045
Comprehensive earnings	220,466	213,310	258,687
Parent Company
Condensed Statement Of Income Captions [Line Items]
Net investment income	2,102	1,412	1,656
Net realized gains	(625)	501	463
Equity in earnings of unconsolidated investees	22,786	10,368	13,592
Selling, general and administrative expenses	(13,330)	(10,265)	(12,686)
Interest on debt	(7,616)	(7,603)	(7,588)
Earnings before income taxes	3,317	(5,587)	(4,563)
Income tax expense	(1,585)	(2,885)	(4,989)
Net earnings (loss) before equity in net earnings of subsidiaries	4,902	(2,702)	426
Equity in net earnings of subsidiaries	274,452	159,793	191,216
Net earnings	279,354	157,091	191,642
Unrealized gains (losses) on securities:
Unrealized holding gains (losses) arising during the period	(1,996)	994	1,727
Less: reclassification adjustment for (gains) losses included in net earnings	500	(390)	(365)
Other comprehensive earnings (loss) - parent only	(1,496)	604	1,362
Equity in other comprehensive earnings (loss) of subsidiaries/investees	(57,392)	55,615	65,683
Net current period other comprehensive earnings (loss)	(58,888)	56,219	67,045
Comprehensive earnings	$ 220,466	$ 213,310	$ 258,687