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Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]          
Cost/Amortized Cost $ 6,311,749   $ 6,311,749   $ 6,073,517
Allowance for credit losses, Period beginning balance 8,781 $ 15,787 3,969 $ 0  
Allowance for credit losses, Period ending balance 6,011 10,395 6,011 10,395  
Interest and dividends due or accrued 46,069   46,069   45,004
Current provision for securities without prior allowance 155 863 3,820 10,930  
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities (2,427) (5,720) (1,102) 0  
Reductions for securities sold (437) (468) (615) (468)  
Reductions for securities identified as intent (or requirement) to sell during the period (61) (67) (61) (67)  
Unrealized Gains 327,387   327,387   397,922
Unrealized Losses (6,725)   (6,725)   (11,542)
Debt Securities, Available-for-sale, Fair Value 6,626,400   6,626,400   6,455,928
Available-for-sale Securities | U.S. Government and Government Agencies [Member]          
Debt Securities, Available-for-sale [Line Items]          
Cost/Amortized Cost 132,506   132,506   110,038
Allowance for credit losses, Period beginning balance     0    
Allowance for credit losses, Period ending balance 0   0    
Unrealized Gains 4,319   4,319   6,239
Unrealized Losses (631)   (631)   (137)
Debt Securities, Available-for-sale, Fair Value 136,194   136,194   116,140
Available-for-sale Securities | Foreign Government [Member]          
Debt Securities, Available-for-sale [Line Items]          
Cost/Amortized Cost 14,578   14,578   16,801
Allowance for credit losses, Period beginning balance 56 21 1 0  
Allowance for credit losses, Period ending balance 49 28 49 28  
Current provision for securities without prior allowance 0 0 49 28  
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities (7) 7 (1) 0  
Reductions for securities sold 0 0 0 0  
Reductions for securities identified as intent (or requirement) to sell during the period 0 0 0 0  
Unrealized Gains 819   819   1,569
Unrealized Losses (73)   (73)   (3)
Debt Securities, Available-for-sale, Fair Value 15,275   15,275   18,366
Available-for-sale Securities | Obligations of States and Political Subdivisions [Member]          
Debt Securities, Available-for-sale [Line Items]          
Cost/Amortized Cost 1,119,039   1,119,039   1,159,588
Allowance for credit losses, Period beginning balance 201 29 4 0  
Allowance for credit losses, Period ending balance 38 17 38 17  
Current provision for securities without prior allowance 0 15 25 17  
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities (163) (27) 9 0  
Reductions for securities sold 0 0 0 0  
Reductions for securities identified as intent (or requirement) to sell during the period 0 0 0 0  
Unrealized Gains 81,003   81,003   87,564
Unrealized Losses (103)   (103)   (11)
Debt Securities, Available-for-sale, Fair Value 1,199,901   1,199,901   1,247,137
Available-for-sale Securities | Corporate Securities [Member]          
Debt Securities, Available-for-sale [Line Items]          
Cost/Amortized Cost 2,325,858   2,325,858   2,152,203
Allowance for credit losses, Period beginning balance 6,166 13,412 2,782 0  
Allowance for credit losses, Period ending balance 3,477 8,077 3,477 8,077  
Current provision for securities without prior allowance 148 813 2,185 8,539  
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities (2,403) (5,686) (909) 0  
Reductions for securities sold (373) (395) (520) (395)  
Reductions for securities identified as intent (or requirement) to sell during the period (61) (67) (61) (67)  
Unrealized Gains 146,061   146,061   180,971
Unrealized Losses (1,555)   (1,555)   (2,340)
Debt Securities, Available-for-sale, Fair Value 2,466,887   2,466,887   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 1,266,236   1,266,236   1,014,820
Allowance for credit losses, Period beginning balance 1,470 1,565 592 0  
Allowance for credit losses, Period ending balance 1,399 1,389 1,399 1,389  
Current provision for securities without prior allowance 0 27 941 1,447  
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities (70) (145) (116) 0  
Reductions for securities sold (1) (58) (18) (58)  
Reductions for securities identified as intent (or requirement) to sell during the period 0 0 0 0  
Unrealized Gains 20,110   20,110   20,166
Unrealized Losses (2,724)   (2,724)   (7,843)
Debt Securities, Available-for-sale, Fair Value 1,282,223   1,282,223   1,026,551
Available-for-sale Securities | Residential Mortgage Backed Securities [Member]          
Debt Securities, Available-for-sale [Line Items]          
Cost/Amortized Cost 820,650   820,650   999,485
Allowance for credit losses, Period beginning balance 864 722 561 0  
Allowance for credit losses, Period ending balance 1,034 831 1,034 831  
Current provision for securities without prior allowance 3 0 618 846  
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities 230 124 (68) 0  
Reductions for securities sold (63) (15) (77) (15)  
Reductions for securities identified as intent (or requirement) to sell during the period 0 0 0 0  
Unrealized Gains 35,655   35,655   53,065
Unrealized Losses (774)   (774)   (201)
Debt Securities, Available-for-sale, Fair Value 854,497   854,497   1,051,788
Available-for-sale Securities | Commercial Mortgage Backed Securities [Member]          
Debt Securities, Available-for-sale [Line Items]          
Cost/Amortized Cost 632,882   632,882   620,582
Allowance for credit losses, Period beginning balance 24 38 29 0  
Allowance for credit losses, Period ending balance 14 53 14 53  
Current provision for securities without prior allowance 4 8 2 53  
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities (14) 7 (17) 0  
Reductions for securities sold 0 0 0 0  
Reductions for securities identified as intent (or requirement) to sell during the period 0 $ 0 0 $ 0  
Unrealized Gains 39,420   39,420   48,348
Unrealized Losses (865)   (865)   (1,007)
Debt Securities, Available-for-sale, Fair Value 671,423   671,423   667,894
Available-for-sale Securities [Member]          
Debt Securities, Available-for-sale [Line Items]          
Interest and dividends due or accrued $ 44,900   $ 44,900   $ 43,800