Basis of Presentation and Summary of Significant Accounting Policies (Tables)
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9 Months Ended |
Sep. 30, 2026 |
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| Accounting Policies [Abstract] |
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| Schedule of Goodwill and Intangible Assets |
| | | | | | | | | September 30, | | December 31, | (in thousands) | | 2026 | | 2025 | 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 |
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| 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, 2026 | | Ended September 30, 2025 | | | 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 | | $ | 81,009 | | | 91,795 | | $ | 0.88 | | $ | 124,610 | | | 91,849 | | $ | 1.36 | Effect of Dilutive Securities | | | | | | | | | | | | | | | | | | | Stock options and restricted stock units | | | — | | | 456 | | | | | | — | | | 461 | | | | Diluted EPS | | | | | | | | | | | | | | | | | | | Earnings available to common shareholders | | $ | 81,009 | | | 92,251 | | $ | 0.88 | | $ | 124,610 | | | 92,310 | | $ | 1.35 | Anti-dilutive securities excluded from diluted EPS | | | | | | 1,008 | | | | | | | | | 819 | | | | | | | | | | | | | | | | | | | | | | | | | For the Nine Months | | For the Nine Months | | | Ended September 30, 2026 | | Ended September 30, 2025 | | | 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 | | $ | 303,922 | | | 91,881 | | $ | 3.31 | | $ | 312,160 | | | 91,816 | | $ | 3.40 | Effect of Dilutive Securities | | | | | | | | | | | | | | | | | | | Stock options and restricted stock units | | | — | | | 378 | | | | | | — | | | 644 | | | | Diluted EPS | | | | | | | | | | | | | | | | | | | Earnings available to common shareholders | | $ | 303,922 | | | 92,259 | | $ | 3.29 | | $ | 312,160 | | | 92,460 | | $ | 3.38 | Anti-dilutive securities excluded from diluted EPS | | | | | | 1,092 | | | | | | | | | 209 | | | |
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| 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 | | 2026 | | 2025 | | 2026 | | 2025 | Beginning balance | | $ | (115,754) | | $ | (124,992) | | $ | (88,074) | | $ | (173,723) | Other comprehensive earnings (loss) before reclassifications | | | (118,861) | | | 29,431 | | | (147,493) | | | 77,253 | Amounts reclassified from accumulated other comprehensive earnings | | | 1,052 | | | (1,757) | | | 2,004 | | | (848) | Net current-period other comprehensive earnings (loss) | | $ | (117,809) | | $ | 27,674 | | $ | (145,489) | | $ | 76,405 | Ending balance | | $ | (233,563) | | $ | (97,318) | | $ | (233,563) | | $ | (97,318) | Balance of securities for which an allowance for credit losses has been recognized in net earnings | | | | | | | | $ | 1,989 | | $ | 1,715 |
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| 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) | | 2026 | | 2025 | | 2026 | | 2025 | | | Statement of Earnings | Unrealized gains and losses on available-for-sale securities | | $ | (1,273) | | $ | 2,878 | | $ | (2,843) | | $ | 1,775 | | | Net realized gains (losses) | | | | (59) | | | (654) | | | 306 | | | (702) | | | Credit gains (losses) presented within net realized gains | | | $ | (1,332) | | $ | 2,224 | | $ | (2,537) | | $ | 1,073 | | | Earnings (loss) before income taxes | | | | 280 | | | (467) | | | 533 | | | (225) | | | Income tax (expense) benefit | | | $ | (1,052) | | $ | 1,757 | | $ | (2,004) | | $ | 848 | | | Net earnings (loss) |
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