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Summary of Significant Accounting Policies - Effects of Reinsurance (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Premiums Written |  |  |
Direct | $ 4,558,500 | $ 4,048,500 | $ 3,907,500
Net | 4,460,000 | 3,980,000 | 3,860,000
Premiums Earned |  |  |
Direct | 4,368,342 | 4,022,611 | 3,793,664
Ceded | (109,445) | (80,950) | (65,017)
Assumed | 15,481 | 10,821 | 13,301
Net | 4,274,378 | 3,952,482 | 3,741,948
Property & Casualty |  |  |
Premiums Written |  |  |
Direct | 4,531,942 | 4,018,057 | 3,877,476
Ceded | (109,644) | (81,256) | (65,521)
Assumed | 15,353 | 10,743 | 13,361
Net | 4,437,651 | 3,947,544 | 3,825,316
Premiums Earned |  |  |
Direct | 4,339,333 | 3,993,771 | 3,764,626
Ceded | (109,445) | (80,950) | (65,017)
Assumed | 15,481 | 10,821 | 13,301
Net | $ 4,245,369 | $ 3,923,642 | $ 3,712,910

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