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Subordinated Notes (Details) - USD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2014
Debt Instrument [Line Items]        
Outstanding subordinated notes $ 436,506,000 $ 436,095,000    
Proceeds, net of underwriting discount $ 0 296,617,000 $ 0  
Stated interest rate 2.43%      
Costs incurred in connection with the issuance of subordinated notes   3,300,000   $ 1,300,000
4.85% subordinated notes maturing in June 2029        
Debt Instrument [Line Items]        
Proceeds, net of underwriting discount   296,700,000    
5.00% subordinated notes maturing in June 2029        
Debt Instrument [Line Items]        
Proceeds, net of underwriting discount       139,100,000
Subordinated debt        
Debt Instrument [Line Items]        
Unamortized balances of costs for issuance of subordinated notes $ 3,500,000      
Subordinated debt | 4.85% subordinated notes maturing in June 2029        
Debt Instrument [Line Items]        
Debt instrument, face amount   $ 300,000,000.0    
Stated interest rate   4.85%    
Subordinated debt | 5.00% subordinated notes maturing in June 2029        
Debt Instrument [Line Items]        
Debt instrument, face amount       $ 140,000,000.0
Stated interest rate       5.00%