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Securities - Amortized cost and fair value by contractual maturity (Details) - USD ($)
 3 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Dec. 31, 2023
Amortized Cost      
Due in one year or less $ 64,365,000    
Due after one year through five years 36,550,000    
Due after five years through ten years 468,529,000    
Due after ten years 1,877,145,000    
Total 2,446,589,000   $ 2,487,440,000
Fair Value      
Due in one year or less 63,576,000    
Due after one year through five years 32,413,000    
Due after five years through ten years 399,174,000    
Due after ten years 1,521,404,000    
Fair Value 2,016,567,000   2,084,736,000
Amortized Cost      
Due in one year or less 157,891,000    
Due after one year through five years 262,765,000    
Due after five years through ten years 1,295,555,000    
Due after ten years 3,712,383,000    
Amortized Cost 5,428,594,000   5,561,006,000
Fair Value      
Due in one year or less 155,556,000    
Due after one year through five years 246,685,000    
Due after five years through ten years 1,111,329,000    
Due after ten years 3,084,830,000    
Fair Value 4,598,400,000   $ 4,784,388,000
Information with respect to sales of held-to-maturity      
Proceeds from sales of investment securities held to maturity $ 0 $ 0  
Information with respect to sales of available-for-sale securities      
Gross gains   1,300,000  
Gross losses   $ (1,300,000)