Basis of Presentation and Summary of Significant Accounting Policies (Tables)

9 Months Ended

Sep. 30, 2025

Accounting Policies [Abstract]

Schedule of Goodwill and Intangible Assets

	September 30,		December 31,
(in thousands)	2025		2024
Goodwill
Surety	$	40,816	$	40,816
Casualty		5,246		5,246
Total goodwill	$	46,062	$	46,062
Indefinite-lived intangibles		7,500		7,500
Total goodwill and intangibles	$	53,562	$	53,562

Schedule of Reconciliation of Numerator and Denominator of the Basic and Diluted Earnings Per Share Computations

	For the Three Months						For the Three Months
	Ended September 30, 2025						Ended September 30, 2024
	Income		Shares		Per Share		Income		Shares		Per Share
(in thousands, except per share data)	(Numerator)		(Denominator)		Amount		(Numerator)		(Denominator)		Amount
Basic EPS
Earnings available to common shareholders	$	124,610		91,849	$	1.36	$	95,027		91,559	$	1.04
Effect of Dilutive Securities
Stock options and restricted stock units		—		461				—		882
Diluted EPS
Earnings available to common shareholders	$	124,610		92,310	$	1.35	$	95,027		92,441	$	1.03
Anti-dilutive securities excluded from diluted EPS				819						48
	For the Nine Months						For the Nine Months
	Ended September 30, 2025						Ended September 30, 2024
	Income		Shares		Per Share		Income		Shares		Per Share
(in thousands, except per share data)	(Numerator)		(Denominator)		Amount		(Numerator)		(Denominator)		Amount
Basic EPS
Earnings available to common shareholders	$	312,160		91,816	$	3.40	$	304,919		91,467	$	3.33
Effect of Dilutive Securities
Stock options and restricted stock units		—		644				—		903
Diluted EPS
Earnings available to common shareholders	$	312,160		92,460	$	3.38	$	304,919		92,370	$	3.30
Anti-dilutive securities excluded from diluted EPS				209						48

Schedule of Changes in the Balance of Each Component of Accumulated Other Comprehensive Earnings (Loss)

(in thousands)	For the Three Months				For the Nine Months
	Ended September 30,				Ended September 30,
Unrealized Gains (Losses) on Available-for-Sale Securities	2025		2024		2025		2024
Beginning balance	$	(124,992)	$	(186,817)	$	(173,723)	$	(166,303)
Other comprehensive earnings (loss) before reclassifications		29,431		79,210		77,253		57,364
Amounts reclassified from accumulated other comprehensive earnings		(1,757)		1,083		(848)		2,415
Net current-period other comprehensive earnings (loss)	$	27,674	$	80,293	$	76,405	$	59,779
Ending balance	$	(97,318)	$	(106,524)	$	(97,318)	$	(106,524)
Balance of securities for which an allowance for credit losses has been recognized in net earnings					$	1,715	$	1,813

Schedule of Effects of Reclassifications out of Accumulated Other Comprehensive Earnings (Loss)

	Amount Reclassified from Accumulated Other
(in thousands)	Comprehensive Earnings (Loss)
	For the Three Months				For the Nine Months
Component of Accumulated	Ended September 30,				Ended September 30,				Affected line item in the
Other Comprehensive Earnings (Loss)	2025		2024		2025		2024		Statement of Earnings
Unrealized gains and losses on available-for-sale securities	$	2,878	$	(1,365)	$	1,775	$	(3,129)	Net realized gains (losses)
		(654)		(6)		(702)		72	Credit gains (losses) presented within net realized gains
	$	2,224	$	(1,371)	$	1,073	$	(3,057)	Earnings (loss) before income taxes
		(467)		288		(225)		642	Income tax (expense) benefit
	$	1,757	$	(1,083)	$	848	$	(2,415)	Net earnings (loss)