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Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) - USD ($) $ in Thousands | Sep. 30, 2023 | Dec. 31, 2022
--- | --- | ---
AFS fixed income securities |  | $ 7,027,104 | $ 6,612,107
Equity securities |  | 125,618 | 162,000
Short-term investments |  | 315,026 | 440,456
Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 7,027,104 | 6,612,107
Equity securities |  | 125,618 | 162,000
Short-term investments |  | 315,026 | 440,456
Total assets measured at fair value |  | 7,467,748 | 7,214,563
US Government and Government Agencies [Member] |  |  |
AFS fixed income securities |  | 226,729 | 189,239
US Government and Government Agencies [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 226,729 | 189,239
Foreign Government [Member] |  |  |
AFS fixed income securities |  | 9,316 | 9,608
Foreign Government [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 9,316 | 9,608
Obligations of States and Political Subdivisions [Member] |  |  |
AFS fixed income securities |  | 614,751 | 918,018
Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 614,751 | 918,018
Corporate Securities [Member] |  |  |
AFS fixed income securities |  | 2,440,135 | 2,335,025
Corporate Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 2,440,135 | 2,335,025
Collateralized Loan Obligations and Other Asset Backed Securities [Member] |  |  |
AFS fixed income securities |  | 1,713,715 | 1,485,973
Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 1,713,715 | 1,485,973
Residential Mortgage Backed Securities [Member] |  |  |
AFS fixed income securities |  | 1,384,459 | 1,059,832
Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 1,384,459 | 1,059,832
Commercial Mortgage Backed Securities [Member] |  |  |
AFS fixed income securities |  | 637,999 | 614,412
Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 637,999 | 614,412
Common Stock [Member] | Fair Value, Recurring [Member] |  |  |
Equity securities | [1] | 123,942 | 160,355
Preferred Stock [Member] | Fair Value, Recurring [Member] |  |  |
Equity securities |  | 1,676 | 1,645
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 67,185 | 109,240
Equity securities |  | 20,717 | 57,491
Short-term investments |  | 303,193 | 418,199
Total assets measured at fair value |  | 391,095 | 584,930
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | US Government and Government Agencies [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 67,185 | 109,240
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 0 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 0 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 0 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 0 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 0 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 0 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Common Stock [Member] | Fair Value, Recurring [Member] |  |  |
Equity securities | [1] | 19,041 | 55,846
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Preferred Stock [Member] | Fair Value, Recurring [Member] |  |  |
Equity securities |  | 1,676 | 1,645
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 6,482,046 | 6,154,509
Equity securities |  | 0 | 0
Short-term investments |  | 11,833 | 22,257
Total assets measured at fair value |  | 6,493,879 | 6,176,766
Significant Other Observable Inputs (Level 2) [Member] | US Government and Government Agencies [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 159,544 | 79,999
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 9,316 | 9,608
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 608,108 | 911,357
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 2,179,656 | 2,147,045
Significant Other Observable Inputs (Level 2) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 1,503,308 | 1,332,631
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 1,384,459 | 1,059,832
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 637,655 | 614,037
Significant Other Observable Inputs (Level 2) [Member] | Common Stock [Member] | Fair Value, Recurring [Member] |  |  |
Equity securities | [1] | 0 | 0
Significant Other Observable Inputs (Level 2) [Member] | Preferred Stock [Member] | Fair Value, Recurring [Member] |  |  |
Equity securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 477,873 | 348,358
Equity securities |  | 933 | 897
Short-term investments |  | 0 | 0
Total assets measured at fair value |  | 478,806 | 349,255
Significant Unobservable Inputs (Level 3) [Member] | US Government and Government Agencies [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 6,643 | 6,661
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 260,479 | 187,980
Significant Unobservable Inputs (Level 3) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 210,407 | 153,342
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS fixed income securities |  | 344 | 375
Significant Unobservable Inputs (Level 3) [Member] | Common Stock [Member] | Fair Value, Recurring [Member] |  |  |
Equity securities | [1] | 933 | 897
Significant Unobservable Inputs (Level 3) [Member] | Preferred Stock [Member] | Fair Value, Recurring [Member] |  |  |
Equity securities |  | 0 | 0
Fair Value Measured at Net Asset Value Per Share |  |  |
Equity securities | [1] | $ 104,000 | $ 103,600
[1] | Investments amounting to $104.0 million at September 30, 2023, and $103.6 million at December 31, 2022, were measured at fair value using the net asset value per share (or its practical expedient) and have not been classified in the fair value hierarchy. These investments are not redeemable and the timing of liquidations of the underlying assets is unknown at each reporting period. The fair value amounts in this table are intended to permit reconciliation of the fair value hierarchy to total assets measured at fair value.

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