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Investments (Available-For-Sale Securities Disclosure) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]    
Cost/Amortized Cost $ 6,073,517 $ 5,879,986
Allowance for credit losses, Period beginning balance 0  
Interest and dividends due or accrued 45,004 44,846
Current provision for securities without prior allowance 5,042  
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities 0  
Reduction for securities sold (974)  
Reduction for securities identified as intent (or requirement) to sell during the period (99)  
Unrealized Gains 397,922 223,372
Unrealized Losses (11,542) (7,738)
Debt Securities, Available-for-sale, Fair Value 6,455,928 6,095,620
Allowance for credit losses, ending balance at December 31, 2020 3,969  
Available-for-sale Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Interest and dividends due or accrued 43,800  
U.S. Government and Government Agencies [Member] | Available-for-sale Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Cost/Amortized Cost 110,038 112,680
Unrealized Gains 6,239 3,506
Unrealized Losses (137) 0
Debt Securities, Available-for-sale, Fair Value 116,140 116,186
Allowance for credit losses, ending balance at December 31, 2020 0  
Foreign Government [Member] | Available-for-sale Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Cost/Amortized Cost 16,801 18,011
Allowance for credit losses, Period beginning balance 0  
Current provision for securities without prior allowance 19  
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities 0  
Reduction for securities sold (18)  
Reduction for securities identified as intent (or requirement) to sell during the period 0  
Unrealized Gains 1,569 533
Unrealized Losses (3) (2)
Debt Securities, Available-for-sale, Fair Value 18,366 18,542
Allowance for credit losses, ending balance at December 31, 2020 1  
Obligations of States and Political Subdivisions [Member] | Available-for-sale Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Cost/Amortized Cost 1,159,588 1,168,185
Allowance for credit losses, Period beginning balance 0  
Current provision for securities without prior allowance 4  
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities 0  
Reduction for securities sold 0  
Reduction for securities identified as intent (or requirement) to sell during the period 0  
Unrealized Gains 87,564 62,175
Unrealized Losses (11) (270)
Debt Securities, Available-for-sale, Fair Value 1,247,137 1,230,090
Allowance for credit losses, ending balance at December 31, 2020 4  
Corporate Securities [Member] | Available-for-sale Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Cost/Amortized Cost 2,152,203 1,866,881
Allowance for credit losses, Period beginning balance 0  
Current provision for securities without prior allowance 3,645  
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities 0  
Reduction for securities sold (781)  
Reduction for securities identified as intent (or requirement) to sell during the period (82)  
Unrealized Gains 180,971 81,906
Unrealized Losses (2,340) (1,310)
Debt Securities, Available-for-sale, Fair Value 2,328,052 1,947,477
Allowance for credit losses, ending balance at December 31, 2020 2,782  
Collateralized Loan Obligations and Other Asset-Backed Securities [Member] | Available-for-sale Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Cost/Amortized Cost 1,014,820 790,517
Allowance for credit losses, Period beginning balance 0  
Current provision for securities without prior allowance 722  
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities 0  
Reduction for securities sold (113)  
Reduction for securities identified as intent (or requirement) to sell during the period (17)  
Unrealized Gains 20,166 7,929
Unrealized Losses (7,843) (5,434)
Debt Securities, Available-for-sale, Fair Value 1,026,551 793,012
Allowance for credit losses, ending balance at December 31, 2020 592  
Residential Mortgage Backed Securities [Member] | Available-for-sale Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Cost/Amortized Cost 999,485 1,409,003
Allowance for credit losses, Period beginning balance 0  
Current provision for securities without prior allowance 623  
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities 0  
Reduction for securities sold (62)  
Reduction for securities identified as intent (or requirement) to sell during the period 0  
Unrealized Gains 53,065 43,421
Unrealized Losses (201) (455)
Debt Securities, Available-for-sale, Fair Value 1,051,788 1,451,969
Allowance for credit losses, ending balance at December 31, 2020 561  
Commercial Mortgage Backed Securities [Member] | Available-for-sale Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Cost/Amortized Cost 620,582 514,709
Allowance for credit losses, Period beginning balance 0  
Current provision for securities without prior allowance 29  
Increase or (Decrease) on securities with prior allowance, excluding intent (or Requirements) to sell securities 0  
Reduction for securities sold 0  
Reduction for securities identified as intent (or requirement) to sell during the period 0  
Unrealized Gains 48,348 23,902
Unrealized Losses (1,007) (267)
Debt Securities, Available-for-sale, Fair Value 667,894 $ 538,344
Allowance for credit losses, ending balance at December 31, 2020 $ 29