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Borrowed Funds - Narrative (Details) - USD ($)
 6 Months Ended 12 Months Ended
May 03, 2024
Jun. 30, 2025
Jun. 30, 2024
Dec. 31, 2024
Short-Term Debt [Line Items]        
Short-term borrowings   $ 650,000,000   $ 365,000,000
Outstanding amount for short-term debt   393,800,000   422,500,000
Unused funds for short-term debt   4,100,000,000    
Securities and loans pledged   3,200,000,000   3,100,000,000
Issuance of common stock   135,000 $ 1,778,000  
Subordinated debentures owed to unconsolidated subsidiary trusts   17,140,000   $ 17,013,000
SFG Capital Trust I        
Short-Term Debt [Line Items]        
Subordinated debentures owed to unconsolidated subsidiary trusts $ 3,600,000      
SFG Capital Trust II        
Short-Term Debt [Line Items]        
Subordinated debentures owed to unconsolidated subsidiary trusts 7,700,000      
SFG Capital Trust III        
Short-Term Debt [Line Items]        
Subordinated debentures owed to unconsolidated subsidiary trusts 8,300,000      
SFG Capital Trust I        
Short-Term Debt [Line Items]        
SFG capital trust issued 3,500,000      
Issuance of common stock 109,000      
SFG Capital Trust II        
Short-Term Debt [Line Items]        
SFG capital trust issued 7,500,000      
Issuance of common stock 232,000      
SFG Capital Trust III        
Short-Term Debt [Line Items]        
SFG capital trust issued 8,000,000.0      
Issuance of common stock $ 248,000      
Subordinated debentures owed to unconsolidated subsidiary trusts | SFG Capital Trust I        
Short-Term Debt [Line Items]        
Basis points 3.45%      
Subordinated debentures owed to unconsolidated subsidiary trusts | SFG Capital Trust II        
Short-Term Debt [Line Items]        
Basis points 2.80%      
Subordinated debentures owed to unconsolidated subsidiary trusts | SFG Capital Trust III        
Short-Term Debt [Line Items]        
Basis points 1.45%      
Subordinated Debentures, Issued In Fourth Quarter Of 2021 | Subordinated debentures        
Short-Term Debt [Line Items]        
Debt instrument, face amount $ 75,000,000.0      
Debt instrument, fair value 61,500,000      
Amortization of debt discount (premium) $ 13,500,000      
Subordinated debentures, net   67,600,000    
Number of maturity (in years) 5 years      
Tier 2 capital reduction (as a percent) 20.00%      
Subordinated borrowing bears interest (as a percent) 3.25%      
Basis points 2.30%      
Debt instrument, term (in years) 10 years      
Number of years not pre payable issuance (in years) 5 years      
Subordinated Debentures, Issued In Third Quarter Of 2020 | Subordinated debentures        
Short-Term Debt [Line Items]        
Debt instrument, face amount $ 30,000,000      
Debt instrument, fair value 29,800,000      
Amortization of debt discount (premium) $ 200,000      
Subordinated debentures, net   $ 30,000,000    
Number of maturity (in years) 5 years      
Tier 2 capital reduction (as a percent) 20.00%      
Subordinated borrowing bears interest (as a percent) 5.00%      
Basis points 4.87%      
Debt instrument, term (in years) 10 years      
Number of years not pre payable issuance (in years) 5 years      
SFG Capital Trust I        
Short-Term Debt [Line Items]        
Subsidiary, ownership percentage, parent (as a percent) 100.00%      
SFG Capital Trust II        
Short-Term Debt [Line Items]        
Subsidiary, ownership percentage, parent (as a percent) 100.00%      
SFG Capital Trust III        
Short-Term Debt [Line Items]        
Subsidiary, ownership percentage, parent (as a percent) 100.00%      
Minimum        
Short-Term Debt [Line Items]        
Interest rate for short-term debt   4.40%   4.43%
Maximum        
Short-Term Debt [Line Items]        
Interest rate for short-term debt   4.50%   4.57%