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Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) - USD ($) $ in Thousands | Mar. 31, 2021 | Dec. 31, 2020
--- | --- | ---
AFS Fixed Income Securities |  | $ 6,521,641 | $ 6,455,928
Equity Securities |  | 324,255 | 310,367
Short-term investments |  | 337,807 | 409,852
Net Asset Value | [1] | 53,500 | 46,800
Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 6,521,641 | 6,455,928
Equity Securities |  | 324,255 | 310,367
Short-term investments |  | 337,807 | 409,852
Total assets measured at fair value |  | 7,183,703 | 7,176,147
U.S. Government and Government Agencies [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 136,274 | 116,140
Foreign Government [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 19,702 | 18,366
Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 1,214,093 | 1,247,137
Corporate Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 2,345,791 | 2,328,052
Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 1,202,065 | 1,026,551
Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 947,540 | 1,051,788
Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 656,176 | 667,894
Common Stock [Member] | Fair Value, Recurring [Member] |  |  |
Equity Securities | [1] | 322,557 | 308,632
Preferred Stock [Member] | Fair Value, Recurring [Member] |  |  |
Equity Securities |  | 1,698 | 1,735
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 58,540 | 40,960
Equity Securities |  | 270,715 | 263,581
Short-term investments |  | 337,075 | 405,400
Total assets measured at fair value |  | 666,330 | 709,941
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 |  | 58,540 | 40,960
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] | 269,017 | 261,846
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Preferred Stock [Member] | Fair Value, Recurring [Member] |  |  |
Equity Securities |  | 1,698 | 1,735
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 6,300,994 | 6,284,999
Equity Securities |  | 0 | 0
Short-term investments |  | 732 | 4,452
Total assets measured at fair value |  | 6,301,726 | 6,289,451
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 77,734 | 75,180
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 19,702 | 18,366
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 1,206,197 | 1,244,243
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 2,256,469 | 2,257,352
Significant Other Observable Inputs (Level 2) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 1,137,176 | 970,176
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 947,540 | 1,051,788
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 656,176 | 667,894
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 |  | 162,107 | 129,969
Equity Securities |  | 0 | 0
Short-term investments |  | 0 | 0
Total assets measured at fair value |  | 162,107 | 129,969
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 |  | 7,896 | 2,894
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 89,322 | 70,700
Significant Unobservable Inputs (Level 3) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 64,889 | 56,375
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 |  | 0 | 0
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 $53.5 million at March 31, 2021, and $46.8 million at December 31, 2020, were measured at fair value using net asset value per share (or its practical expedient) and are not 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 presented in this table are intended to permit reconciliation of the fair value hierarchy to total assets measured at fair value.

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