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Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) - USD ($)
$ in Thousands
 6 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Percentage increase in long-term debt 1.00%  
Investments Measured at Fair Value Using Net Asset Value $ 27,100 $ 23,700
Available-for-sale Securities, Fixed Income Securities 5,137,653 5,162,522
Equity securities, available-for-sale   182,705
Equity securities 176,578  
Total AFS securities 5,137,653 5,345,227
Short-term investments 164,118 165,555
Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Fixed Income Securities 5,137,653 5,162,522
Equity securities, available-for-sale   182,705
Equity securities 176,578  
Total AFS securities   5,345,227
Short-term investments 164,118 165,555
Total assets measured at fair value 5,478,349 5,510,782
U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Fixed Income Securities 38,896 49,740
Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Fixed Income Securities 17,982 18,555
Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Fixed Income Securities 1,253,667 1,582,970
Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Fixed Income Securities 1,614,484 1,617,468
Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Fixed Income Securities 771,597 795,458
Commercial Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Fixed Income Securities 451,593 383,449
Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Fixed Income Securities 989,434 714,882
Common Stock [Member] | Fair Value, Measurements, Recurring [Member]    
Equity securities, available-for-sale [1]   167,757
Equity securities [1] 172,157  
Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member]    
Equity securities, available-for-sale   14,948
Equity securities 4,421  
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] 22,479 24,652
Equity securities, available-for-sale [2]   153,588
Equity securities [2] 149,459  
Total AFS securities [2]   178,240
Short-term investments [2] 163,120 165,555
Total assets measured at fair value [2] 335,058 343,795
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] 22,479 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] 145,038  
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] 4,421  
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Fixed Income Securities [2] 5,111,985 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] 998 0
Total assets measured at fair value [2] 5,112,983 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] 16,417 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] 17,982 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] 1,253,667 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] 1,614,484 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] 768,408 795,458
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] 451,593 376,895
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] 989,434 714,882
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 3,189 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 3,189 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 3,189 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 $27.1 million at June 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.