Operating Segment Information - Summary of Selected Information by Operating Segment (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2022	Jun. 30, 2021
Revenues
Net premiums earned	$ 282,810		$ 241,000		$ 551,962	$ 469,595
Net investment income	18,472		16,661		36,355	33,085
Net realized gains	12,804		36,463		18,392	50,613
Net unrealized gains (losses) on equity securities	(100,994)		3,956		(128,804)	32,118
Consolidated revenue	213,092		298,080		477,905	585,411
Net Earnings
Net underwriting income	55,956		36,591		115,434	66,508
Net investment income	18,472		16,661		36,355	33,085
Net realized gains	12,804		36,463		18,392	50,613
Net unrealized gains (losses) on equity securities	(100,994)		3,956		(128,804)	32,118
General corporate expense and interest on debt	(4,446)		(5,590)		(9,819)	(10,833)
Equity in earnings of unconsolidated investees	11,654		13,940		20,413	20,364
Earnings (loss) before income taxes	(6,554)		102,021		51,971	191,855
Income tax expense (benefit)	(4,315)		20,206		6,287	37,028
Net earnings (loss)	(2,239)	$ 47,923	81,815	$ 73,012	45,684	154,827
Casualty Segment
Revenues
Net premiums earned	177,123		156,195		348,879	304,965
Net Earnings
Net underwriting income	21,442		26,438		49,089	51,305
Property Segment
Revenues
Net premiums earned	74,690		55,865		142,130	107,507
Net Earnings
Net underwriting income	26,105		8,879		48,581	7,874
Surety Segment
Revenues
Net premiums earned	30,997		28,940		60,953	57,123
Net Earnings
Net underwriting income	$ 8,409		$ 1,274		$ 17,764	$ 7,329