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Investment Securities - Investments' Gross Unrealized Losses and Fair Value Aggregated by Investment Category and Length of Time in Continuous Unrealized Loss Position (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Fair Value    
Less than 12 Months $ 799,578 $ 15,019
12 Months or Longer 501 0
Total 800,079 15,019
Gross Unrealized Losses    
Less than 12 Months 16,153 349
12 Months or Longer 8 0
Total 16,161 349
State and municipal    
Fair Value    
Less than 12 Months 273,742 5,368
12 Months or Longer 501 0
Total 274,243 5,368
Gross Unrealized Losses    
Less than 12 Months 3,938 246
12 Months or Longer 8 0
Total 3,946 246
U.S. Government-sponsored mortgage-backed securities    
Fair Value    
Less than 12 Months 525,836 9,651
12 Months or Longer 0 0
Total 525,836 9,651
Gross Unrealized Losses    
Less than 12 Months 12,215 103
12 Months or Longer 0 0
Total $ 12,215 $ 103