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Fair Value Measurements (Quantitative Disclosures at Fair Value Assets) (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments Net Asset Value | $ 37,300 | $ 23,700
Debt Securities, Available-for-sale | 5,273,100 | 5,162,522
Equity securities | 147,639 |
Equity securities, AFS |  | 182,705
Total AFS securities |  | 5,345,227
Short-term investments | 323,864 | 165,555
Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Available-for-sale | 5,273,100 | 5,162,522
Equity securities | 147,639 |
Equity securities, AFS |  | 182,705
Total AFS securities |  | 5,345,227
Short-term investments | 323,864 | 165,555
Total Assets | 5,744,603 | 5,510,782
U.S. Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Available-for-sale | 121,310 | 49,740
Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Available-for-sale | 23,131 | 18,555
Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Available-for-sale | 1,138,469 | 1,582,970
Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Available-for-sale | 1,617,408 | 1,617,468
Collateralized Loan Obligations and Other Asset-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Available-for-sale | 717,362 | 795,458
Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Available-for-sale | 527,078 | 383,449
Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Available-for-sale | 1,128,342 | 714,882
Common Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Equity securities | 144,727 |
Equity securities, AFS |  | 167,757
Nonredeemable Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Equity securities | 2,912 |
Equity securities, AFS |  | 14,948
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Available-for-sale | 78,381 | 24,652
Equity securities | 110,309 |
Equity securities, AFS |  | 153,588
Total AFS securities |  | 178,240
Short-term investments | 321,370 | 165,555
Total Assets | 510,060 | 343,795
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | U.S. Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Available-for-sale | 78,381 | 24,652
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Foreign Government [Member] | Fair Value, Measurements, 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, Measurements, 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, Measurements, 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, Measurements, 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] | Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Measurements, 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, Measurements, 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, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Equity securities | 107,397 |
Equity securities, AFS |  | 138,640
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Nonredeemable Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Equity securities | 2,912 |
Equity securities, AFS |  | 14,948
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Available-for-sale | 5,187,310 | 5,131,316
Equity securities | 0 |
Equity securities, AFS |  | 0
Total AFS securities |  | 5,131,316
Short-term investments | 2,494 | 0
Total Assets | 5,189,804 | 5,131,316
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Available-for-sale | 42,929 | 25,088
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Available-for-sale | 23,131 | 18,555
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Available-for-sale | 1,138,469 | 1,582,970
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Available-for-sale | 1,617,408 | 1,617,468
Significant Other Observable Inputs (Level 2) [Member] | Collateralized Loan Obligations and Other Asset-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Available-for-sale | 709,953 | 795,458
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Available-for-sale | 527,078 | 376,895
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Available-for-sale | 1,128,342 | 714,882
Significant Other Observable Inputs (Level 2) [Member] | Common Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Equity securities | 0 |
Equity securities, AFS |  | 0
Significant Other Observable Inputs (Level 2) [Member] | Nonredeemable Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Equity securities | 0 |
Equity securities, AFS |  | 0
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Available-for-sale | 7,409 | 6,554
Equity securities | 0 |
Equity securities, AFS |  | 5,398
Total AFS securities |  | 11,952
Short-term investments | 0 | 0
Total Assets | 7,409 | 11,952
Significant Unobservable Inputs (Level 3) [Member] | U.S. Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, 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, Measurements, 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, Measurements, 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, Measurements, 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] | Collateralized Loan Obligations and Other Asset-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Available-for-sale | 7,409 | 0
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt Securities, Available-for-sale | 0 | 6,554
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, 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, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Equity securities | 0 |
Equity securities, AFS |  | 5,398
Significant Unobservable Inputs (Level 3) [Member] | Nonredeemable Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Equity securities | $ 0 |
Equity securities, AFS |  | $ 0

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