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Investments - Narrative (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]          
Debt Securities, Available-for-sale, Amortized Cost $ 1,211,293   $ 1,211,293   $ 882,682
Maximum percentage of shareholders' equity security holdings held of one issuer 10.00%   10.00%   10.00%
Securities pledged $ 582,300   $ 582,300   $ 525,800
Mortgage-backed securities, weighted average life     4 years    
Accrued interest receivable, held-to-maturity securities 3,500   $ 3,500   3,600
Provision for credit losses on held-to-maturity securities 200        
Sale of debt securities, available-for-sale 27,100 $ 20,000 27,135 $ 20,221  
Allowance for credit losses on held-to-maturity debt securities (640)   (640)   (449)
Other investments, at cost 59,156   59,156   48,764
Des Moines          
Debt Securities, Available-for-sale [Line Items]          
Other investments, at cost $ 12,100   $ 12,100   $ 10,800