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Fair Value Measurements (Quantitative Disclosures at Fair Value Assets) (Details) - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Investments Net Asset Value |  | $ 37,800 | $ 37,300
Debt Securities, Available-for-sale |  | 6,095,620 | 5,273,100
Equity securities |  | 72,937 | 147,639
Short-term investments |  | 282,490 | 323,864
Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 6,095,620 | 5,273,100
Equity securities |  | 72,937 | 147,639
Short-term investments |  | 282,490 | 323,864
Total Assets |  | 6,451,047 | 5,744,603
U.S. Government and Government Agencies and Authorities [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 116,186 | 121,310
Foreign Government [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 18,542 | 23,131
Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 1,230,090 | 1,138,469
Corporate Securities [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 1,947,477 | 1,617,408
Collateralized Loan Obligations and Other Asset-Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 793,012 | 717,362
Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 538,344 | 527,078
Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 1,451,969 | 1,128,342
Common Stock [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Equity securities | [1] | 69,900 | 144,727
Nonredeemable Preferred Stock [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Equity securities |  | 3,037 | 2,912
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 44,718 | 78,381
Equity securities |  | 35,182 | 110,309
Short-term investments |  | 265,306 | 321,370
Total Assets |  | 345,206 | 510,060
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | U.S. Government and Government Agencies and Authorities [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 41,083 | 78,381
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 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] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 0 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 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] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 3,635 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 0 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 0 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Common Stock [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Equity securities | [1] | 32,145 | 107,397
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Nonredeemable Preferred Stock [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Equity securities |  | 3,037 | 2,912
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 6,016,817 | 5,187,310
Equity securities |  | 0 | 0
Short-term investments |  | 17,184 | 2,494
Total Assets |  | 6,034,001 | 5,189,804
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Government Agencies and Authorities [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 75,103 | 42,929
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 18,542 | 23,131
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 1,230,090 | 1,138,469
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 1,930,426 | 1,617,408
Significant Other Observable Inputs (Level 2) [Member] | Collateralized Loan Obligations and Other Asset-Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 772,343 | 709,953
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 538,344 | 527,078
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 1,451,969 | 1,128,342
Significant Other Observable Inputs (Level 2) [Member] | Common Stock [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Equity securities | [1] | 0 | 0
Significant Other Observable Inputs (Level 2) [Member] | Nonredeemable Preferred Stock [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Equity securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 34,085 | 7,409
Equity securities |  | 0 | 0
Short-term investments |  | 0 | 0
Total Assets |  | 34,085 | 7,409
Significant Unobservable Inputs (Level 3) [Member] | U.S. Government and Government Agencies and Authorities [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 17,051 | 0
Significant Unobservable Inputs (Level 3) [Member] | Collateralized Loan Obligations and Other Asset-Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 17,034 | 7,409
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt Securities, Available-for-sale |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Common Stock [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Equity securities | [1] | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Nonredeemable Preferred Stock [Member] | Fair Value, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Equity securities |  | $ 0 | $ 0
[1] | In accordance with ASU 2015-07, investments amounting to $37.8 million and $37.3 million at December 31, 2019 and December 31, 2018, respectively, 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. 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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