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Investment Securities Available-for-Sale - Expected maturities for residential mortgage backed securities (Details) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Amortized Cost [Abstract]    
Allowance for Credit Losses $ (78) $ (167)
Amortized Cost 1,370,927 1,129,057
Estimated Fair Value [Abstract]    
Estimated Fair Value 1,368,909 1,150,885
U. S. agency securities    
Amortized Cost [Abstract]    
One year or less 96,891 53,916
After one year through five years 125,593 110,083
Five years through ten years 57,663 17,087
Amortized Cost 280,147 181,087
Estimated Fair Value [Abstract]    
One year or less 94,564 53,906
After one year through five years 125,151 110,777
Five years through ten years 55,820 17,240
Residential mortgage backed securities    
Amortized Cost [Abstract]    
Residential mortgage backed securities 934,615 811,328
Amortized Cost 934,615 811,328
Estimated Fair Value [Abstract]    
Residential mortgage backed securities 932,619 825,001
Municipal bonds    
Amortized Cost [Abstract]    
One year or less 3,343 4,329
After one year through five years 26,564 26,622
Five years through ten years 71,323 69,309
After ten years 2,000 2,000
Amortized Cost 103,230 102,259
Estimated Fair Value [Abstract]    
One year or less 3,327 4,348
After one year through five years 28,042 28,272
Five years through ten years 73,698 73,389
After ten years 2,062 2,121
Corporate bonds    
Amortized Cost [Abstract]    
One year or less 500 5,218
After one year through five years 37,823 22,189
After ten years 14,612 6,976
Amortized Cost 52,935 34,383
Estimated Fair Value [Abstract]    
One year or less 500 5,220
After one year through five years 38,392 23,267
After ten years $ 14,812 $ 7,511