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Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018
--- | --- | ---
AFS Fixed Income Securities |  | $ 5,567,358 | $ 5,273,100
Short-term investments |  | 290,653 | 323,864
Net Asset Value |  | 40,000 | 23,700
Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 5,567,358 | 5,273,100
Equity securities |  | 163,016 | 147,639
Short-term investments |  | 290,653 | 323,864
Total assets measured at fair value |  | 6,021,027 | 5,744,603
U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 127,089 | 121,310
Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 20,526 | 23,131
Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 1,150,243 | 1,138,469
Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 1,678,015 | 1,617,408
Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 722,240 | 717,362
Commercial Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 568,200 | 527,078
Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 1,301,045 | 1,128,342
Common Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Equity securities | [1] | 160,011 | 144,727
Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Equity securities |  | 3,005 | 2,912
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 68,645 | 78,381
Equity securities | [2] | 122,982 | 110,309
Short-term investments | [2] | 289,232 | 321,370
Total assets measured at fair value | [2] | 480,859 | 510,060
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 68,645 | 78,381
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 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] |  |  |
AFS Fixed Income Securities | [2] | 0 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 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] |  |  |
AFS Fixed Income Securities | [2] | 0 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 0 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 0 | 0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Common Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Equity securities | [1],[2] | 119,977 | 107,397
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Equity securities | [2] | 3,005 | 2,912
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 5,451,002 | 5,187,310
Equity securities | [2] | 0 | 0
Short-term investments | [2] | 1,421 | 2,494
Total assets measured at fair value | [2] | 5,452,423 | 5,189,804
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 58,444 | 42,929
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 20,526 | 23,131
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 1,150,243 | 1,138,469
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 1,661,596 | 1,617,408
Significant Other Observable Inputs (Level 2) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 690,948 | 709,953
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 568,200 | 527,078
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities | [2] | 1,301,045 | 1,128,342
Significant Other Observable Inputs (Level 2) [Member] | Common Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Equity securities | [1],[2] | 0 | 0
Significant Other Observable Inputs (Level 2) [Member] | Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Equity securities | [2] | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 47,711 | 7,409
Equity securities |  | 0 | 0
Short-term investments |  | 0 | 0
Total assets measured at fair value |  | 47,711 | 7,409
Significant Unobservable Inputs (Level 3) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 16,419 | 0
Significant Unobservable Inputs (Level 3) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 31,292 | 7,409
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
AFS Fixed Income Securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Common Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Equity securities | [1] | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Equity securities |  | $ 0 | $ 0
[1] | Investments amounting to $40.0 million at March 31, 2019, and $23.7 million at December 31, 2018, were measured at fair value using 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.
[2] | There were no transfers of securities between Level 1 and Level 2.

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