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Investments (Narrative) (Details)
3 Months Ended
Mar. 31, 2021
USD ($)
item
Dec. 31, 2020
USD ($)
item
Schedule of Available-for-sale Securities [Line Items]    
Fair Value $ 422,178,000 $ 396,940,000
Number of temporarily impaired securities | item 205 102
Unrealized Losses $ 5,552,000 $ 701,000
Restricted stock 468,000 468,000
Equity securities $ 444,000 391,000
Corporate Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Number of temporarily impaired securities | item 205  
Temporarily impaired fair value $ 178,600,000  
Unrealized Losses 5,600,000 4,900,000
Public Funds And Trust Deposits [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Fair Value $ 129,100,000 $ 137,400,000