Operating Segment Information - Summary of Selected Information by Operating Segment (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Revenues
Net premiums earned	$ 241,000		$ 208,734		$ 469,595	$ 424,316
Net investment income	16,661		16,917		33,085	34,695
Net realized gains (losses)	36,463		(2,109)		50,613	13,043
Net unrealized gains (losses) on equity securities	3,956		74,705		32,118	(55,690)
Consolidated revenue	298,080		298,247		585,411	416,364
Net Earnings
Net underwriting income	36,591		24,163		66,508	41,402
Net investment income	16,661		16,917		33,085	34,695
Net realized gains (losses)	36,463		(2,109)		50,613	13,043
Net unrealized gains (losses) on equity securities	3,956		74,705		32,118	(55,690)
General corporate expense and interest on debt	(5,590)		(3,897)		(10,833)	(7,549)
Equity in earnings of unconsolidated investees	13,940		5,100		20,364	9,614
Earnings before income taxes	102,021		114,879		191,855	35,515
Income tax expense	20,206		22,713		37,028	4,616
Net earnings	81,815	$ 73,012	92,166	$ (61,267)	154,827	30,899
Casualty Segment
Revenues
Net premiums earned	156,195		135,215		304,965	278,635
Net Earnings
Net underwriting income	26,438		9,733		51,305	8,410
Property Segment
Revenues
Net premiums earned	55,865		45,387		107,507	89,735
Net Earnings
Net underwriting income	8,879		6,167		7,874	16,075
Surety Segment
Revenues
Net premiums earned	28,940		28,132		57,123	55,946
Net Earnings
Net underwriting income	$ 1,274		$ 8,263		$ 7,329	$ 16,917