Operating Segment Information - Operating Results by Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Revenue
Net premiums earned	$ 398,345	$ 360,676
Net investment income	36,726	32,847
Net realized gains	14,912	5,994
Net unrealized gains (losses) on equity securities	(42,318)	45,314
Consolidated revenue	407,665	444,831
Less: Expenses
Losses and settlement expenses	177,238	143,824
Insurance operating expenses	26,874	28,703
Earnings before income taxes	78,631	159,991
Reconciliation of earnings before income taxes
Net investment income	36,726	32,847
Net realized gains	14,912	5,994
Net unrealized gains (losses) on equity securities	(42,318)	45,314
Interest expense on debt	(1,335)	(1,618)
General corporate expense	(2,948)	(5,010)
Equity in earnings of unconsolidated investees	3,048	4,769
Casualty Segment
Revenue
Net premiums earned	229,048	198,276
Property Segment
Revenue
Net premiums earned	132,544	129,411
Surety Segment
Revenue
Net premiums earned	36,753	32,989
Operating Segments
Revenue
Net premiums earned	398,345	360,676
Less: Expenses
Earnings before income taxes	70,546	77,695
Operating Segments | Casualty Segment
Revenue
Net premiums earned	229,048	198,276
Consolidated revenue	229,048	198,276
Less: Expenses
Losses and settlement expenses	145,835	109,472
Amortization of deferred acquisition costs	42,903	36,835
Other policy acquisition costs	22,669	21,599
Insurance operating expenses	15,570	16,696
Earnings before income taxes	2,071	13,674
Reconciliation of earnings before income taxes
Depreciation and amortization expense	1,505	1,409
Operating Segments | Property Segment
Revenue
Net premiums earned	132,544	129,411
Consolidated revenue	132,544	129,411
Less: Expenses
Losses and settlement expenses	32,725	32,577
Amortization of deferred acquisition costs	26,976	26,219
Other policy acquisition costs	7,957	4,415
Insurance operating expenses	7,971	8,484
Earnings before income taxes	56,915	57,716
Reconciliation of earnings before income taxes
Depreciation and amortization expense	498	452
Operating Segments | Surety Segment
Revenue
Net premiums earned	36,753	32,989
Consolidated revenue	36,753	32,989
Less: Expenses
Losses and settlement expenses	(1,322)	1,775
Amortization of deferred acquisition costs	12,602	11,276
Other policy acquisition costs	10,580	10,110
Insurance operating expenses	3,333	3,523
Earnings before income taxes	11,560	6,305
Reconciliation of earnings before income taxes
Depreciation and amortization expense	310	259
Corporate
Revenue
Net investment income	36,726	32,847
Net realized gains	14,912	5,994
Net unrealized gains (losses) on equity securities	(42,318)	45,314
Reconciliation of earnings before income taxes
Net investment income	36,726	32,847
Net realized gains	14,912	5,994
Net unrealized gains (losses) on equity securities	(42,318)	45,314
Interest expense on debt	(1,335)	(1,618)
General corporate expense	(2,948)	(5,010)
Equity in earnings of unconsolidated investees	$ 3,048	$ 4,769