Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2024

Accounting Policies [Abstract]

Schedule of Summarized Financial Information of Equity Method Investments

(in millions)	2024		2023
Total assets	$	1,102	$	944
Total liabilities		844		691
Total equity		258		253

Schedule of Goodwill and Intangible Assets

(in thousands)	2024		2023
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

			Weighted Average
	Income		Shares	Per Share
(in thousands, except per share data)	(Numerator)		(Denominator)	Amount
For the year ended December 31, 2024
Basic EPS
Income available to common shareholders	$	345,779	91,529	$	3.78
Stock options		—	922
Diluted EPS
Income available to common shareholders and assumed conversions	$	345,779	92,451	$	3.74
Anti-dilutive options excluded from diluted EPS			48
For the year ended December 31, 2023
Basic EPS
Income available to common shareholders	$	304,611	91,191	$	3.34
Stock options		—	964
Diluted EPS
Income available to common shareholders and assumed conversions	$	304,611	92,155	$	3.31
Anti-dilutive options excluded from diluted EPS			345
For the year ended December 31, 2022
Basic EPS
Income available to common shareholders	$	583,411	90,735	$	6.43
Stock options		—	854
Diluted EPS
Income available to common shareholders and assumed conversions	$	583,411	91,589	$	6.37
Anti-dilutive options excluded from diluted EPS			590

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

Unrealized Gains/Losses on Available-for-Sale Securities	For the Year Ended December 31,
(in thousands)	2024		2023		2022
Beginning balance	$	(166,303)	$	(229,076)	$	49,826
Other comprehensive earnings before reclassifications		(11,501)		59,922		(281,189)
Amounts reclassified from accumulated other comprehensive earnings		4,081		2,851		2,287
Net current period other comprehensive earnings (loss)	$	(7,420)	$	62,773	$	(278,902)
Ending balance	$	(173,723)	$	(166,303)	$	(229,076)
Balance of securities for which an allowance for credit losses has not been recognized in net earnings	$	1,492	$	1,224	$	1,693

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

Amount Reclassified from Accumulated Other Comprehensive Earnings
(in thousands)
Component of Accumulated		For the Year Ended December 31,					Affected line item in the
Other Comprehensive Earnings	2024		2023		2022		Consolidated Statement of Earnings
Unrealized gains and losses on available-for-sale securities	$	(5,276)	$	(3,641)	$	(2,997)	Net realized gains (losses)
		110		32		102	Credit gains (losses) presented within net realized gains
	$	(5,166)	$	(3,609)	$	(2,895)	Earnings (loss) before income taxes
		1,085		758		608	Income tax (expense) benefit
	$	(4,081)	$	(2,851)	$	(2,287)	Net earnings (loss)