Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2021

Accounting Policies [Abstract]

Schedule of Summarized Financial Information of Equity Method Investments

(in millions)	2021		2020
Total assets	$	1,172.6	$	837.2
Total liabilities		687.8		473.2
Total equity		484.8		364.0

Schedule of Goodwill and Intangible Assets

(in thousands)	2021		2020
Goodwill
Surety	$	40,816	$	40,816
Casualty		5,246		5,246
Total goodwill	$	46,062	$	46,062
Intangibles
State insurance licenses	$	7,500	$	7,500
Definite-lived intangibles, net of accumulated amortization of $4,035 at 12/31/21 and $3,878 at 12/31/20		-		157
Total intangibles	$	7,500	$	7,657
Total goodwill and intangibles	$	53,562	$	53,719

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, 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
For the year ended December 31, 2019
Basic EPS
Income available to common shareholders	$	191,642		44,734	$	4.28
Stock options		—		523
Diluted EPS
Income available to common shareholders and assumed conversions	$	191,642		45,257	$	4.23
Anti-dilutive options excluded from diluted EPS				65

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

Unrealized Gains/Losses on Available-for-Sale Securities	For the Year Ended December 31,
(in thousands)	2021			2020			2019
Beginning balance	$	108,714		$	52,473		$	(14,572	)
Cumulative-effect adjustment of ASU 2016-13		—			22			—
Adjusted beginning balance	$	108,714		$	52,495		$	(14,572	)
Other comprehensive earnings before reclassifications		(57,454	)		58,986			69,560
Amounts reclassified from accumulated other comprehensive earnings		(1,434	)		(2,767	)		(2,515	)
Net current period other comprehensive earnings (loss)	$	(58,888	)	$	56,219		$	67,045
Ending balance	$	49,826		$	108,714		$	52,473
Balance of securities for which an allowance for credit losses has not been recognized in net earnings	$	124		$	470		$	—

Schedule of Effects of Reclassifications out of Accumulated Other Comprehensive Earnings

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	2021			2020			2019			Consolidated Statement of Earnings
Unrealized gains and losses on available-for-sale securities	$	1,859		$	3,872		$	3,184		Net realized gains
		(44	)		(369	)		—		Credit losses presented within net realized gains
	$	1,815		$	3,503		$	3,184		Earnings before income taxes
		(381	)		(736	)		(669	)	Income tax expense
	$	1,434		$	2,767		$	2,515		Net earnings