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Investments - Narrative (Details) - USD ($)
 3 Months Ended 6 Months Ended
Jun. 30, 2025
Jun. 30, 2024
Jun. 30, 2025
Jun. 30, 2024
Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]          
Cumulative net unamortized unrealized gain $ 8,842,000   $ 8,842,000   $ 1,187,000
Maximum percentage of shareholders' equity security holdings held of one issuer 10.00%   10.00%   10.00%
Securities pledged $ 1,700,000,000   $ 1,700,000,000   $ 1,500,000,000
Mortgage-backed securities, weighted average life     6 years    
Allowance for credit losses on available-for-sale securities 0   $ 0   0
Accrued interest receivable, held-to-maturity securities 10,700,000   10,700,000   8,600,000
Allowance for credit losses on held-to-maturity securities 295,000   295,000   257,000
Other investments, at cost 88,598,000   88,598,000   72,784,000
Sale of debt securities, available-for-sale 0 $ 0 9,500,000 $ 100,000  
Reclassified to Held to Maturity          
Debt Securities, Available-for-sale [Line Items]          
Cumulative net unamortized unrealized gain 9,200,000   9,200,000   10,800,000
Des Moines          
Debt Securities, Available-for-sale [Line Items]          
Other investments, at cost $ 22,600,000   $ 22,600,000   $ 8,700,000