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Investments (Summary of Gross Unrealized Gain (Loss) on Investments) (Details) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Investments, Unrealized Loss Position, Fair Value    
Investments, Fair Value 12 months or less $ 80,592 $ 309,764
Investments, Fair Value Longer than 12 months 48,264 169,544
Investments, Total Fair Value 128,856 479,308
Investments, Unrealized Losses    
Investments, Unrealized Losses 12 months or less (5,166) (12,496)
Investments, Unrealized Losses Longer than 12 months (3,537) (7,289)
Investments, Total Unrealized Losses (8,703) (19,785)
Debt Securities [Member]    
Debt Securities, Unrealized Loss Position, Fair Value    
Debt securities, Fair Value 12 months or less 71,054 277,635
Debt securities, Fair Value Longer than 12 months 42,864 165,069
Debt securities, Total Fair Value 113,918 442,704
Debt securities, Unrealized Losses    
Debt securities, Unrealized Losses 12 months or less (891) (6,182)
Debt securities, Unrealized Losses Longer than 12 months (296) (2,482)
Debt securities, Total Unrealized Losses 1,187 8,664
Debt Securities [Member] | U.S. Treasury Securities and Obligations of U.S. Government [Member]    
Debt Securities, Unrealized Loss Position, Fair Value    
Debt securities, Fair Value 12 months or less   18,902
Debt securities, Fair Value Longer than 12 months 23,142 28,201
Debt securities, Total Fair Value 23,142 47,103
Debt securities, Unrealized Losses    
Debt securities, Unrealized Losses 12 months or less   (181)
Debt securities, Unrealized Losses Longer than 12 months (22) (327)
Debt securities, Total Unrealized Losses 22 508
Debt Securities [Member] | Corporate Bonds [Member]    
Debt Securities, Unrealized Loss Position, Fair Value    
Debt securities, Fair Value 12 months or less 29,172 117,450
Debt securities, Fair Value Longer than 12 months 13,305 100,060
Debt securities, Total Fair Value 42,477 217,510
Debt securities, Unrealized Losses    
Debt securities, Unrealized Losses 12 months or less (142) (907)
Debt securities, Unrealized Losses Longer than 12 months (32) (695)
Debt securities, Total Unrealized Losses 174 1,602
Debt Securities [Member] | Collateralized Corporate Bank Loans [Member]    
Debt Securities, Unrealized Loss Position, Fair Value    
Debt securities, Fair Value 12 months or less 33,961 120,410
Debt securities, Fair Value Longer than 12 months 4,782 4,931
Debt securities, Total Fair Value 38,743 125,341
Debt securities, Unrealized Losses    
Debt securities, Unrealized Losses 12 months or less (636) (4,938)
Debt securities, Unrealized Losses Longer than 12 months (151) (332)
Debt securities, Total Unrealized Losses 787 5,270
Debt Securities [Member] | Municipal Bonds [Member]    
Debt Securities, Unrealized Loss Position, Fair Value    
Debt securities, Fair Value 12 months or less 6,537 14,281
Debt securities, Fair Value Longer than 12 months 1,036 25,891
Debt securities, Total Fair Value 7,573 40,172
Debt securities, Unrealized Losses    
Debt securities, Unrealized Losses 12 months or less (48) (96)
Debt securities, Unrealized Losses Longer than 12 months (7) (742)
Debt securities, Total Unrealized Losses 55 838
Debt Securities [Member] | Mortgage Backed [Member]    
Debt Securities, Unrealized Loss Position, Fair Value    
Debt securities, Fair Value 12 months or less 1,384 6,592
Debt securities, Fair Value Longer than 12 months 599 5,986
Debt securities, Total Fair Value 1,983 12,578
Debt securities, Unrealized Losses    
Debt securities, Unrealized Losses 12 months or less (65) (60)
Debt securities, Unrealized Losses Longer than 12 months (84) (386)
Debt securities, Total Unrealized Losses 149 446
Equity Securities [Member]    
Equity Securities, Unrealized Loss Position, Fair Value    
Equity securities, Fair Value 12 months or less 9,538 30,981
Equity securities, Fair Value Longer than 12 months 2,391 4,475
Equity securities, Total Fair Value 11,929 35,456
Equity Securities, Unrealized Loss    
Equity securities, Unrealized Losses 12 months or less (4,275) (3,699)
Equity securities, Unrealized Losses Longer than 12 months (2,487) (4,807)
Equity securities, Total Unrealized Losses (6,762) (8,506)
Other Investments [Member]    
Equity Securities, Unrealized Loss Position, Fair Value    
Equity securities, Fair Value 12 months or less   1,148
Equity securities, Fair Value Longer than 12 months 3,009  
Equity securities, Total Fair Value 3,009 1,148
Equity Securities, Unrealized Loss    
Equity securities, Unrealized Losses Longer than 12 months (754) (2,615)
Equity securities, Total Unrealized Losses $ (754) $ (2,615)