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Investments - Narrative (Details) - USD ($)
 3 Months Ended 6 Months Ended
Jun. 30, 2022
Jun. 30, 2021
Jun. 30, 2022
Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]        
Debt Securities, Available-for-sale, Amortized Cost $ 1,658,412,000   $ 1,658,412,000 $ 1,358,960,000
Cumulative net unamortized unrealized gain $ 127,000   $ 127,000 15,247,000
Maximum percentage of shareholders' equity security holdings held of one issuer 10.00%   10.00%  
Securities pledged $ 680,100,000   $ 680,100,000 752,700,000
Mortgage-backed securities, weighted average life     5 years  
Allowance for credit losses on available-for-sale securities 0   $ 0 0
Accrued interest receivable, held-to-maturity securities 4,400,000   4,400,000 3,400,000
Allowance for credit losses on held-to-maturity debt securities (707,000)   (707,000) (614,000)
Sale of debt securities, available-for-sale 0 $ 0    
Other investments, at cost 61,274,000   61,274,000 59,896,000
Reclassified to Held to Maturity        
Debt Securities, Available-for-sale [Line Items]        
Debt Securities, Available-for-sale, Amortized Cost 116,900,000   116,900,000  
Cumulative net unamortized unrealized gain 19,400,000   19,400,000 21,000,000
Des Moines        
Debt Securities, Available-for-sale [Line Items]        
Other investments, at cost $ 12,000,000   $ 12,000,000 $ 12,100,000