Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2023

Accounting Policies [Abstract]

Schedule of Summarized Financial Information of Equity Method Investments

(in millions)	2023		2022
Total assets	$	944	$	906
Total liabilities		691		690
Total equity		253		216

Schedule of Goodwill and Intangible Assets

(in thousands)	2023		2022
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, 2023
Basic EPS
Income available to common shareholders	$	304,611	45,596	$	6.68
Stock options		—	481
Diluted EPS
Income available to common shareholders and assumed conversions	$	304,611	46,077	$	6.61
Anti-dilutive options excluded from diluted EPS			173
For the year ended December 31, 2022
Basic EPS
Income available to common shareholders	$	583,411	45,368	$	12.86
Stock options		—	426
Diluted EPS
Income available to common shareholders and assumed conversions	$	583,411	45,794	$	12.74
Anti-dilutive options excluded from diluted EPS			295
For the year ended December 31, 2021
Basic EPS
Income available to common shareholders	$	279,354	45,230	$	6.18
Stock options		—	482
Diluted EPS
Income available to common shareholders and assumed conversions	$	279,354	45,712	$	6.11
Anti-dilutive options excluded from diluted EPS			214

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)	2023		2022		2021
Beginning balance	$	(229,076)	$	49,826	$	108,714
Other comprehensive earnings before reclassifications		59,922		(281,189)		(57,454)
Amounts reclassified from accumulated other comprehensive earnings		2,851		2,287		(1,434)
Net current period other comprehensive earnings (loss)	$	62,773	$	(278,902)	$	(58,888)
Ending balance	$	(166,303)	$	(229,076)	$	49,826
Balance of securities for which an allowance for credit losses has not been recognized in net earnings	$	1,224	$	1,693	$	124

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	2023		2022		2021		Consolidated Statement of Earnings
Unrealized gains and losses on available-for-sale securities	$	(3,641)	$	(2,997)	$	1,859	Net realized gains (losses)
		32		102		(44)	Credit gains (losses) presented within net realized gains
	$	(3,609)	$	(2,895)	$	1,815	Earnings (loss) before income taxes
		758		608		(381)	Income tax (expense) benefit
	$	(2,851)	$	(2,287)	$	1,434	Net earnings (loss)