Operating Segment Information - Summary of Selected Information by Operating Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Revenues
Net premiums earned	$ 228,595	$ 215,582
Net investment income	16,424	17,778
Net realized gains	14,150	15,152
Net unrealized gains (losses) on equity securities	28,162	(130,395)
Consolidated revenue	287,331	118,117
Net Earnings
Net underwriting income	29,917	17,239
Net investment income	16,424	17,778
Net realized gains	14,150	15,152
Net unrealized gains (losses) on equity securities	28,162	(130,395)
General corporate expense and interest on debt	(5,243)	(3,652)
Equity in earnings of unconsolidated investees	6,424	4,514
Earnings (loss) before income taxes	89,834	(79,364)
Income tax expense (benefit)	16,822	(18,097)
Net earnings (loss)	73,012	(61,267)
Casualty Segment
Revenues
Net premiums earned	148,770	143,420
Net Earnings
Net underwriting income	24,867	(1,323)
Property Segment
Revenues
Net premiums earned	51,642	44,348
Net Earnings
Net underwriting income	(1,005)	9,908
Surety Segment
Revenues
Net premiums earned	28,183	27,814
Net Earnings
Net underwriting income	$ 6,055	$ 8,654