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Investment Securities - Amortized Cost of Investment Securities Held to Maturity (Details) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities $ 1,295,534 $ 1,227,668
Aaa    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 109,365  
Aa1    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 84,954  
Aa2    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 107,436  
Aa3    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 72,969  
A1    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 65,578  
A2    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 19,074  
A3    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 1,065  
Baa2    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 528  
Non-rated    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 834,565  
State and municipal    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 624,553 $ 619,927
State and municipal | Aaa    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 48,689  
State and municipal | Aa1    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 84,954  
State and municipal | Aa2    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 107,436  
State and municipal | Aa3    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 72,969  
State and municipal | A1    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 65,578  
State and municipal | A2    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 19,074  
State and municipal | A3    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 1,065  
State and municipal | Baa2    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 528  
State and municipal | Non-rated    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 224,260  
Other    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 670,981  
Other | Aaa    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 60,676  
Other | Aa1    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 0  
Other | Aa2    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 0  
Other | Aa3    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 0  
Other | A1    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 0  
Other | A2    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 0  
Other | A3    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 0  
Other | Baa2    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities 0  
Other | Non-rated    
Schedule of Held-to-maturity Securities [Line Items]    
Held to maturity securities $ 610,305