Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2022

Accounting Policies [Abstract]

Schedule of Summarized Financial Information of Equity Method Investments

(in millions)	2022		2021
Total assets	$	906.4	$	1,172.6
Total liabilities		689.9		687.8
Total equity		216.5		484.8

Schedule of Goodwill and Intangible Assets

(in thousands)	2022		2021
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, 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
For the year ended December 31, 2020
Basic EPS
Income available to common shareholders	$	157,091	45,000	$	3.49
Stock options		—	376
Diluted EPS
Income available to common shareholders and assumed conversions	$	157,091	45,376	$	3.46
Anti-dilutive options excluded from diluted EPS			384

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)	2022		2021		2020
Beginning balance	$	49,826	$	108,714	$	52,473
Cumulative-effect adjustment of ASU 2016-13		—		—		22
Adjusted beginning balance	$	49,826	$	108,714	$	52,495
Other comprehensive earnings before reclassifications		(281,189)		(57,454)		58,986
Amounts reclassified from accumulated other comprehensive earnings		2,287		(1,434)		(2,767)
Net current period other comprehensive earnings (loss)	$	(278,902)	$	(58,888)	$	56,219
Ending balance	$	(229,076)	$	49,826	$	108,714
Balance of securities for which an allowance for credit losses has not been recognized in net earnings	$	1,693	$	124	$	470

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	2022		2021		2020		Consolidated Statement of Earnings
Unrealized gains and losses on available-for-sale securities	$	(2,997)	$	1,859	$	3,872	Net realized gains
		102		(44)		(369)	Credit losses presented within net realized gains
	$	(2,895)	$	1,815	$	3,503	Earnings before income taxes
		608		(381)		(736)	Income tax expense
	$	(2,287)	$	1,434	$	2,767	Net earnings