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Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) - USD ($) $ in Thousands | Jun. 30, 2022 | Dec. 31, 2021
--- | --- | ---
AFS Fixed Income Securities |  | $ 6,439,278 | $ 6,709,976
Equity Securities |  | 258,515 | 335,537
Short-term investments |  | 289,219 | 447,863
Net Asset Value | [1] | 97,500 | 83,600
Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 6,439,278 | 6,709,976
Equity Securities |  | 258,515 | 335,537
Short-term investments |  | 289,219 | 447,863
Total assets measured at fair value |  | 6,987,012 | 7,493,376
U.S. Government and Government Agencies [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 114,345 | 130,458
Foreign Government [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 15,298 | 15,860
Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 1,066,238 | 1,189,308
Corporate Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 2,276,327 | 2,573,603
Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 1,437,778 | 1,350,814
Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 911,800 | 776,252
Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 617,492 | 673,681
Common Stock [Member] | Fair Value, Recurring [Member] |  |  |
Equity Securities | [1] | 256,773 | 333,449
Preferred Stock [Member] | Fair Value, Recurring [Member] |  |  |
Equity Securities |  | 1,742 | 2,088
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 35,242 | 60,615
Equity Securities |  | 160,985 | 251,934
Short-term investments |  | 288,717 | 442,723
Total assets measured at fair value |  | 484,944 | 755,272
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 35,242 | 60,615
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] | 159,243 | 249,846
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Preferred Stock [Member] | Fair Value, Recurring [Member] |  |  |
Equity Securities |  | 1,742 | 2,088
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 6,113,716 | 6,398,079
Equity Securities |  | 0 | 0
Short-term investments |  | 502 | 5,140
Total assets measured at fair value |  | 6,114,218 | 6,403,219
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 79,103 | 69,843
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 15,298 | 15,860
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 1,059,230 | 1,181,563
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 2,117,038 | 2,459,476
Significant Other Observable Inputs (Level 2) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 1,314,162 | 1,225,905
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 911,800 | 776,007
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 617,085 | 669,425
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 |  | 290,320 | 251,282
Equity Securities |  | 0 | 0
Short-term investments |  | 0 | 0
Total assets measured at fair value |  | 290,320 | 251,282
Significant Unobservable Inputs (Level 3) [Member] | U.S. 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 |  | 7,008 | 7,745
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 159,289 | 114,127
Significant Unobservable Inputs (Level 3) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 123,616 | 124,909
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 0 | 245
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 407 | 4,256
Significant Unobservable Inputs (Level 3) [Member] | Common Stock [Member] | Fair Value, Recurring [Member] |  |  |
Equity Securities | [1] | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Preferred Stock [Member] | Fair Value, Recurring [Member] |  |  |
Equity Securities |  | $ 0 | $ 0
[1] | Investments amounting to $97.5 million at June 30, 2022, and $83.6 million at December 31, 2021, 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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