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Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Investments Net Asset Value $ 25,000 $ 23,700
Available-for-sale Securities, Debt Securities 5,141,637 5,162,522
Equity securities, available-for-sale   182,705
Equity securities 168,842  
Total AFS securities 5,141,637 5,345,227
Short-term investments 182,980 165,555
Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Debt Securities 5,141,637 5,162,522
Equity securities, available-for-sale   182,705
Equity securities 168,842  
Total AFS securities   5,345,227
Short-term investments 182,980 165,555
Total assets measured at fair value 5,493,459 5,510,782
U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Debt Securities 50,949 49,740
Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Debt Securities 18,200 18,555
Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Debt Securities 1,318,160 1,582,970
Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Debt Securities 1,621,722 1,617,468
Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Debt Securities 794,483 795,458
Commercial Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Debt Securities 426,583 383,449
Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Debt Securities 911,540 714,882
Common Stock [Member] | Fair Value, Measurements, Recurring [Member]    
Equity securities, available-for-sale [1]   167,757
Equity securities [1] 164,370  
Preferred Stock [Member] | Fair Value, Measurements, Recurring [Member]    
Equity securities, available-for-sale   14,948
Equity securities 4,472  
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Debt Securities [2] 22,584 24,652
Equity securities, available-for-sale [2]   153,588
Equity securities [2] 143,870  
Total AFS securities [2]   178,240
Short-term investments [2] 181,983 165,555
Total assets measured at fair value [2] 348,437 343,795
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, Debt Securities [2] 22,584 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, Debt 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, Debt 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, Debt 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, Debt 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, Debt 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, Debt 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] 139,398  
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] 4,472  
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Debt Securities [2] 5,119,053 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] 997 0
Total assets measured at fair value [2] 5,120,050 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, Debt Securities [2] 28,365 25,088
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Debt Securities [2] 18,200 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, Debt Securities [2] 1,318,160 1,582,970
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Debt Securities [2] 1,621,722 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, Debt Securities [2] 794,483 795,458
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Debt Securities [2] 426,583 376,895
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Debt Securities [2] 911,540 714,882
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, Debt Securities 0 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 0 11,952
Significant Unobservable Inputs (Level 3) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Debt Securities 0 0
Significant Unobservable Inputs (Level 3) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Debt 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, Debt Securities 0 0
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Debt 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, Debt Securities 0 0
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Debt Securities 0 6,554
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale Securities, Debt 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  
One Point Ninety Eight Percent Borrowing From Federal Home Loan Bank Due April 18, 2018 [Member]    
Federal Home Loan Bank Advances $ 55,000  
[1] Investments amounting to $25.0 million at March 31, 2018, and $23.7 million at December 31, 2017, 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.
[2] There were no transfers of securities between Level 1 and Level 2.