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Segment Information - Summary of Operating Results by Segment (Details) $ in Thousands | 12 Months Ended
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Dec. 31, 2024 USD ($) segment State Subsidiary | Dec. 31, 2023 USD ($) | Dec. 31, 2022 USD ($)
Segment Reporting [Abstract] |  |  |
Number of insurance companies | Subsidiary | 12 |  |
Number of states in which company operates | State | 11 |  |
Number of reportable segments | segment | 1 |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | $ 5,075,456 | $ 4,274,378 | $ 3,952,482
Total incurred losses and loss adjustment expenses | 3,684,511 | 3,517,853 | 3,362,219
Policy acquisition costs | 858,261 | 708,525 | 654,612
Other operating expenses | 327,157 | 279,656 | 279,718
Net investment income | 279,989 | 234,630 | 168,356
Net realized investment gains (losses) | 88,671 | 101,014 | (488,080)
Other income | 31,517 | 19,609 | 10,308
Interest expense | (30,824) | (24,169) | (17,232)
Income (loss) before income taxes | 574,880 | 99,428 | (670,715)
Net income (loss) | 467,953 | 96,336 | (512,672)
Operating Segments |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 5,075,500 | 4,274,400 | 3,952,500
Losses | 3,171,300 | 3,033,100 | 2,864,400
Loss adjustment expenses | 513,200 | 484,800 | 497,800
Total incurred losses and loss adjustment expenses | 3,684,500 | 3,517,900 | 3,362,200
Policy acquisition costs | 858,300 | 708,500 | 654,600
Other operating expenses | 327,200 | 279,700 | 279,700
Underwriting gain (loss) | 205,500 | (231,700) | (344,000)
Net investment income | 280,000 | 234,600 | 168,400
Net realized investment gains (losses) | 88,700 | 101,000 | (488,100)
Other income | 31,500 | 19,700 | 10,200
Interest expense | (30,800) | (24,200) | (17,200)
Income (loss) before income taxes | 574,900 | 99,400 | (670,700)
Net income (loss) | 468,000 | 96,300 | (512,700)
Property & Casualty |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 5,046,099 | 4,245,369 | 3,923,642
Property & Casualty | Operating Segments |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 5,046,100 | 4,245,400 | 3,923,600
Losses | 3,156,400 | 3,019,400 | 2,851,300
Loss adjustment expenses | 511,300 | 482,800 | 495,900
Total incurred losses and loss adjustment expenses | 3,667,700 | 3,502,200 | 3,347,200
Policy acquisition costs | 846,600 | 696,900 | 645,500
Other operating expenses | 324,100 | 276,600 | 276,500
Underwriting gain (loss) | 207,700 | (230,300) | (345,600)
Other | Operating Segments |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 29,400 | 29,000 | 28,900
Losses | 14,900 | 13,700 | 13,100
Loss adjustment expenses | 1,900 | 2,000 | 1,900
Total incurred losses and loss adjustment expenses | 16,800 | 15,700 | 15,000
Policy acquisition costs | 11,700 | 11,600 | 9,100
Other operating expenses | 3,100 | 3,100 | 3,200
Underwriting gain (loss) | $ (2,200) | $ (1,400) | $ 1,600

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