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Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) - USD ($) $ in Thousands | 9 Months Ended |
--- | --- | ---
Sep. 30, 2018 | Dec. 31, 2017
Percentage increase in long-term debt |  | 1.00% |
Available-for-sale Securities, Fixed Income Securities |  | $ 5,190,156 | $ 5,162,522
Equity securities, available-for-sale |  |  | 182,705
Total AFS securities |  | 5,190,156 | 5,345,227
Short-term investments |  | 304,572 | 165,555
Net Asset Value |  | 30,400 | 23,700
Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities |  | 5,190,156 | 5,162,522
Equity securities, available-for-sale |  |  | 182,705
Equity securities |  | 157,867 |
Total AFS securities |  |  | 5,345,227
Short-term investments |  | 304,572 | 165,555
Total assets measured at fair value |  | 5,652,595 | 5,510,782
U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities |  | 96,462 | 49,740
Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities |  | 17,983 | 18,555
Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities |  | 1,164,909 | 1,582,970
Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities |  | 1,649,653 | 1,617,468
Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities |  | 774,769 | 795,458
Commercial Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities |  | 489,549 | 383,449
Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities |  | 996,831 | 714,882
Common Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Equity securities, available-for-sale | [1] |  | 167,757
Equity securities | [1] | 154,845 |
Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Equity securities, available-for-sale |  |  | 14,948
Equity securities |  | 3,022 |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities | [2] | 76,760 | 24,652
Equity securities, available-for-sale | [2] |  | 153,588
Equity securities | [2] | 127,467 |
Total AFS securities | [2] |  | 178,240
Short-term investments | [2] | 303,572 | 165,555
Total assets measured at fair value | [2] | 507,799 | 343,795
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities | [2] | 76,760 | 24,652
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, 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] |  |  |
Available-for-sale Securities, 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] |  |  |
Available-for-sale Securities, 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] |  |  |
Available-for-sale Securities, 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] |  |  |
Available-for-sale Securities, 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] |  |  |
Available-for-sale Securities, 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, available-for-sale | [1],[2] |  | 138,640
Equity securities | [1],[2] | 124,445 |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Equity securities, available-for-sale | [2] |  | 14,948
Equity securities | [2] | 3,022 |
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities | [2] | 5,109,255 | 5,131,316
Equity securities, available-for-sale | [2] |  | 0
Equity securities | [2] | 0 |
Total AFS securities | [2] |  | 5,131,316
Short-term investments | [2] | 1,000 | 0
Total assets measured at fair value | [2] | 5,110,255 | 5,131,316
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities | [2] | 19,702 | 25,088
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities | [2] | 17,983 | 18,555
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities | [2] | 1,164,909 | 1,582,970
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities | [2] | 1,649,653 | 1,617,468
Significant Other Observable Inputs (Level 2) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities | [2] | 770,628 | 795,458
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities | [2] | 489,549 | 376,895
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities | [2] | 996,831 | 714,882
Significant Other Observable Inputs (Level 2) [Member] | Common Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Equity securities, available-for-sale | [1],[2] |  | 0
Equity securities | [1],[2] | 0 |
Significant Other Observable Inputs (Level 2) [Member] | Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Equity securities, available-for-sale | [2] |  | 0
Equity securities | [2] | 0 |
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities |  | 4,141 | 6,554
Equity securities, available-for-sale |  |  | 5,398
Equity securities |  | 0 |
Total AFS securities |  |  | 11,952
Short-term investments |  | 0 | 0
Total assets measured at fair value |  | 4,141 | 11,952
Significant Unobservable Inputs (Level 3) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities |  | 4,141 | 0
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities |  | 0 | 6,554
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Available-for-sale Securities, Fixed Income Securities |  | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] | Common Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Equity securities, available-for-sale | [1] |  | 5,398
Equity securities | [1] | 0 |
Significant Unobservable Inputs (Level 3) [Member] | Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Equity securities, available-for-sale |  |  | $ 0
Equity securities |  | $ 0 |
[1] | Investments amounting to $30.4 million at September 30, 2018, and $23.7 million at December 31, 2017, 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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