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Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]      
Cost/Amortized Cost $ 6,253,077   $ 6,073,517
Allowance for credit losses, Period beginning balance 3,969 $ 0  
Allowance for credit losses, Period ending balance 8,781 15,787  
Interest and dividends due or accrued 46,072   45,004
Current provision for securities without prior allowance 5,667 15,787  
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities (677) 0  
Reductions for securities sold (178) 0  
Reductions for securities identified as intent (or requirement) to sell during the period 0 0  
Unrealized Gains 297,474   397,922
Unrealized Losses (20,129)   (11,542)
Debt Securities, Available-for-sale, Fair Value 6,521,641   6,455,928
Available-for-sale Securities | U.S. Government and Government Agencies [Member]      
Debt Securities, Available-for-sale [Line Items]      
Cost/Amortized Cost 134,725   110,038
Allowance for credit losses, Period beginning balance 0    
Allowance for credit losses, Period ending balance 0    
Unrealized Gains 3,653   6,239
Unrealized Losses (2,104)   (137)
Debt Securities, Available-for-sale, Fair Value 136,274   116,140
Available-for-sale Securities | Foreign Government [Member]      
Debt Securities, Available-for-sale [Line Items]      
Cost/Amortized Cost 18,920   16,801
Allowance for credit losses, Period beginning balance 1 0  
Allowance for credit losses, Period ending balance 56 21  
Current provision for securities without prior allowance 56 21  
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities (1) 0  
Reductions for securities sold 0 0  
Reductions for securities identified as intent (or requirement) to sell during the period 0 0  
Unrealized Gains 974   1,569
Unrealized Losses (136)   (3)
Debt Securities, Available-for-sale, Fair Value 19,702   18,366
Available-for-sale Securities | Obligations of States and Political Subdivisions [Member]      
Debt Securities, Available-for-sale [Line Items]      
Cost/Amortized Cost 1,140,442   1,159,588
Allowance for credit losses, Period beginning balance 4 0  
Allowance for credit losses, Period ending balance 201 29  
Current provision for securities without prior allowance 186 29  
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities 11 0  
Reductions for securities sold 0 0  
Reductions for securities identified as intent (or requirement) to sell during the period 0 0  
Unrealized Gains 74,212   87,564
Unrealized Losses (360)   (11)
Debt Securities, Available-for-sale, Fair Value 1,214,093   1,247,137
Available-for-sale Securities | Corporate Securities [Member]      
Debt Securities, Available-for-sale [Line Items]      
Cost/Amortized Cost 2,239,066   2,152,203
Allowance for credit losses, Period beginning balance 2,782 0  
Allowance for credit losses, Period ending balance 6,166 13,412  
Current provision for securities without prior allowance 4,058 13,412  
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities (527) 0  
Reductions for securities sold (147) 0  
Reductions for securities identified as intent (or requirement) to sell during the period 0 0  
Unrealized Gains 122,761   180,971
Unrealized Losses (9,870)   (2,340)
Debt Securities, Available-for-sale, Fair Value 2,345,791   2,328,052
Available-for-sale Securities | Collateralized Loan Obligations and Other Asset Backed Securities [Member]      
Debt Securities, Available-for-sale [Line Items]      
Cost/Amortized Cost 1,188,670   1,014,820
Allowance for credit losses, Period beginning balance 592 0  
Allowance for credit losses, Period ending balance 1,470 1,565  
Current provision for securities without prior allowance 1,001 1,565  
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities (106) 0  
Reductions for securities sold (17) 0  
Reductions for securities identified as intent (or requirement) to sell during the period 0 0  
Unrealized Gains 18,917   20,166
Unrealized Losses (4,052)   (7,843)
Debt Securities, Available-for-sale, Fair Value 1,202,065   1,026,551
Available-for-sale Securities | Residential Mortgage Backed Securities [Member]      
Debt Securities, Available-for-sale [Line Items]      
Cost/Amortized Cost 906,266   999,485
Allowance for credit losses, Period beginning balance 561 0  
Allowance for credit losses, Period ending balance 864 722  
Current provision for securities without prior allowance 356 722  
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities (39) 0  
Reductions for securities sold (14) 0  
Reductions for securities identified as intent (or requirement) to sell during the period 0 0  
Unrealized Gains 43,367   53,065
Unrealized Losses (1,229)   (201)
Debt Securities, Available-for-sale, Fair Value 947,540   1,051,788
Available-for-sale Securities | Commercial Mortgage Backed Securities [Member]      
Debt Securities, Available-for-sale [Line Items]      
Cost/Amortized Cost 624,988   620,582
Allowance for credit losses, Period beginning balance 29 0  
Allowance for credit losses, Period ending balance 24 38  
Current provision for securities without prior allowance 10 38  
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities (15) 0  
Reductions for securities sold 0 0  
Reductions for securities identified as intent (or requirement) to sell during the period 0 $ 0  
Unrealized Gains 33,590   48,348
Unrealized Losses (2,378)   (1,007)
Debt Securities, Available-for-sale, Fair Value 656,176   667,894
Available-for-sale Securities [Member]      
Debt Securities, Available-for-sale [Line Items]      
Interest and dividends due or accrued $ 44,700   $ 43,800