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Investments (Narrative) (Details)
 9 Months Ended
Sep. 30, 2021
USD ($)
security
item
Dec. 31, 2020
USD ($)
security
Schedule of Available-for-sale Securities [Line Items]    
Fair value $ 545,788,000 $ 396,940,000
Percent of shareholders equity benchmark for investments in a single issuer 10.00%  
Number of debt securities | security 204 102
Fair value of temporarily impaired securities $ 234,800,000 $ 82,950,000
Unrealized Losses 3,465,000 701,000
Total unrealized loss position increased over prior year 2,800,000  
Equity securities 473,000 391,000
Corporate [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Fair value $ 24,956,000 $ 20,288,000
Number of debt securities | security 19 17
Fair value of temporarily impaired securities $ 9,902,000 $ 10,616,000
Unrealized Losses 141,000 155,000
Public Funds And Trust Deposits [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Fair value $ 168,000,000.0 137,400,000
Equity Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Number of equity investments | item 1  
Equity securities $ 473,000 $ 391,000