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Investments (Tables) | 9 Months Ended
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Sep. 30, 2025
Investments, Debt and Equity Securities [Abstract] |
Schedule of Fair Values and Gross Unrealized Gains and Losses | Fair Values and Gross Unrealized Gains and Losses on Investments
| Amortized Cost |  | Gross Unrealized |  | Fair Value
|  | Gains |  | Losses |
As of September 30, 2025 | (In Thousands)
U.S. Treasury securities and obligations of U.S. government agencies | $ | 100,950 |  |  | $ | 2,710 |  |  | $ | (165) |  |  | $ | 103,495 |
Municipal debt securities | 658,725 |  |  | 2,603 |  |  | (30,964) |  |  | 630,364 |
Corporate debt securities | 2,148,007 |  |  | 27,454 |  |  | (61,582) |  |  | 2,113,879 |
Asset-backed securities | 52,559 |  |  | — |  |  | (2,796) |  |  | 49,763 |
Total bonds | 2,960,241 |  |  | 32,767 |  |  | (95,507) |  |  | 2,897,501 |
Short-term investments | 118,053 |  |  | 8 |  |  | (2) |  |  | 118,059 |
Total investments | $ | 3,078,294 |  |  | $ | 32,775 |  |  | $ | (95,509) |  |  | $ | 3,015,560 |
| Amortized Cost |  | Gross Unrealized |  | Fair Value
|  | Gains |  | Losses |
As of December 31, 2024 | (In Thousands)
U.S. Treasury securities and obligations of U.S. government agencies | $ | 115,342 |  |  | $ | 1,207 |  |  | $ | (489) |  |  | $ | 116,060 |
Municipal debt securities | 684,523 |  |  | 589 |  |  | (49,867) |  |  | 635,245 |
Corporate debt securities | 1,949,800 |  |  | 3,981 |  |  | (106,141) |  |  | 1,847,640 |
Asset-backed securities | 44,104 |  |  | 1 |  |  | (2,125) |  |  | 41,980 |
Total bonds | 2,793,769 |  |  | 5,778 |  |  | (158,622) |  |  | 2,640,925 |
Short-term investments | 82,574 |  |  | 42 |  |  | — |  |  | 82,616 |
Total investments | $ | 2,876,343 |  |  | $ | 5,820 |  |  | $ | (158,622) |  |  | $ | 2,723,541 |
Schedule of Corporate Debt Securities by Industry Group | The following table presents a breakdown of the fair value of our corporate debt securities by issuer industry group as of September 30, 2025 and December 31, 2024:
| September 30, 2025 |  | December 31, 2024
Financial | 39 | % |  | 40 | %
Consumer | 28 |  |  | 26 |
Utilities | 11 |  |  | 11 |
Industrial | 9 |  |  | 9 |
Communications | 6 |  |  | 7 |
Technology | 6 |  |  | 7 |
Basic materials | 1 |  |  | — |
Total | 100 | % |  | 100 | %
Schedule of Investments by Maturity | The amortized cost and fair value of available-for-sale securities as of September 30, 2025 and December 31, 2024, by contractual maturity, are shown below. Expected maturities will differ from contractual maturities because issuers may have the right to call or prepay obligations with or without call or prepayment penalties. Because most asset-backed securities provide for periodic payments throughout their lives, they are listed below in a separate category.
As of September 30, 2025 | Amortized Cost |  | Fair Value
| (In Thousands)
Due in one year or less | $ | 327,073 |  |  | $ | 325,628 |
Due after one through five years | 1,767,053 |  |  | 1,721,568 |
Due after five through ten years | 924,819 |  |  | 911,763 |
Due after ten years | 6,790 |  |  | 6,838 |
Asset-backed securities | 52,559 |  |  | 49,763 |
Total investments | $ | 3,078,294 |  |  | $ | 3,015,560 |
As of December 31, 2024 | Amortized Cost |  | Fair Value
| (In Thousands)
Due in one year or less | $ | 261,492 |  |  | $ | 260,132 |
Due after one through five years | 1,549,468 |  |  | 1,479,220 |
Due after five through ten years | 995,717 |  |  | 916,885 |
Due after ten years | 25,562 |  |  | 25,324 |
Asset-backed securities | 44,104 |  |  | 41,980 |
Total investments | $ | 2,876,343 |  |  | $ | 2,723,541 |
Schedule of Aging Unrealized Losses | For those securities in an unrealized loss position, the length of time the securities were in such a position is as follows:
| Less Than Twelve Months |  | Twelve Months or Greater |  | Total
| # of Securities | Fair Value | Unrealized Losses |  | # of Securities | Fair Value | Unrealized Losses |  | # of Securities | Fair Value | Unrealized Losses
As of September 30, 2025 |  | ($ In Thousands)
U.S. Treasury securities and obligations of U.S. government agencies | 1 |  | $ | 125 |  | * |  | 7 |  | $ | 10,819 |  | $ | (165) |  |  | 8 |  | $ | 10,944 |  | $ | (165) |
Municipal debt securities | 1 |  | 893 |  | (3) |  |  | 221 |  | 478,460 |  | (30,961) |  |  | 222 |  | 479,353 |  | (30,964) |
Corporate debt securities | 15 |  | 64,253 |  | (378) |  |  | 170 |  | 866,782 |  | (61,204) |  |  | 185 |  | 931,035 |  | (61,582) |
Asset-backed securities | 3 |  | 13,133 |  | (37) |  |  | 16 |  | 36,630 |  | (2,759) |  |  | 19 |  | 49,763 |  | (2,796) |
Short-term investments | 3 |  | 31,149 |  | (2) |  |  | — |  | — |  | — |  |  | 3 |  | 31,149 |  | (2) |
Total | 23 |  | $ | 109,553 |  | $ | (420) |  |  | 414 |  | $ | 1,392,691 |  | $ | (95,089) |  |  | 437 |  | $ | 1,502,244 |  | $ | (95,509) |
| Less Than Twelve Months |  | Twelve Months or Greater |  | Total
| # of Securities | Fair Value | Unrealized Losses |  | # of Securities | Fair Value | Unrealized Losses |  | # of Securities | Fair Value | Unrealized Losses
As of December 31, 2024 |  | ($ In Thousands)
U.S. Treasury securities and obligations of U.S. government agencies | 4 |  | $ | 8,002 |  | $ | (98) |  |  | 9 |  | $ | 11,510 |  | $ | (391) |  |  | 13 |  | $ | 19,512 |  | $ | (489) |
Municipal debt securities | 25 |  | 110,648 |  | (1,928) |  |  | 220 |  | 471,770 |  | (47,939) |  |  | 245 |  | 582,418 |  | (49,867) |
Corporate debt securities | 68 |  | 366,113 |  | (5,822) |  |  | 226 |  | 1,053,862 |  | (100,319) |  |  | 294 |  | 1,419,975 |  | (106,141) |
Asset-backed securities | — |  | — |  | — |  |  | 19 |  | 41,559 |  | (2,125) |  |  | 19 |  | 41,559 |  | (2,125) |
Short-term investments | 1 |  | 1,551 |  | * |  | — |  | — |  | — |  |  | 1 |  | 1,551 |  | *
Total | 98 |  | $ | 486,314 |  | $ | (7,848) |  |  | 474 |  | $ | 1,578,701 |  | $ | (150,774) |  |  | 572 |  | $ | 2,065,015 |  | $ | (158,622) |
* Amounts not identifiable due to rounding.
Schedule of Net Investment Income | The following table presents the components of net investment income:
| For the three months ended September 30, |  | For the nine months ended September 30,
| 2025 |  | 2024 |  | 2025 |  | 2024
| (In Thousands)
Investment income (1) | $ | 27,127 |  |  | $ | 22,772 |  |  | $ | 76,425 |  |  | $ | 63,423 |
Investment expenses | (354) |  |  | (298) |  |  | (1,017) |  |  | (825) |
Net investment income | $ | 26,773 |  |  | $ | 22,474 |  |  | $ | 75,408 |  |  | $ | 62,598 |
(1) Includes interest income recognized on cash and cash equivalents of $0.9 million and $2.9 million during the three and nine months ended September 30, 2025, respectively, and $1.3 million and $4.2 million during the three and nine months ended September 30, 2024, respectively.
Schedule of Net Realized Investment Gains (Losses) | The following table presents the components of net realized investment gains (losses):
| For the three months ended September 30, |  | For the nine months ended September 30,
| 2025 |  | 2024 |  | 2025 |  | 2024
| (In Thousands)
Gross realized investment gains | $ | 390 |  |  | $ | — |  |  | $ | 414 |  |  | $ | — |
Gross realized investment losses | (69) |  |  | (10) |  |  | (469) |  |  | (10) |
Net realized investment gains (losses) | $ | 321 |  |  | $ | (10) |  |  | $ | (55) |  |  | $ | (10) |

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