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Securities Available for Sale and Held to Maturity - Amortized Cost and Estimated Fair Value of Available for Sale and Held to Maturity Securities (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Schedule of Available For Sale and Held to Maturity Securities [Line Items]    
Securities Available for Sale, Amortized Cost $ 3,053,294 $ 1,959,773
Securities Available for Sale, Gross Unrealized Gains 21,015 33,889
Securities Available for Sale, Gross Unrealized (Losses) (16,704) (1,847)
Securities Available for Sale, Estimated Fair Value 3,057,605 1,991,815
Securities Held to Maturity, Amortized Cost 394,905 538,072
Securities Held to Maturity, Gross Unrealized Gains 16,272 25,101
Securities Held to Maturity, Gross Unrealized (Losses) (45) (58)
Securities Held to Maturity, Estimated Fair Value 411,132 563,115
U.S. Treasury Securities [Member]    
Schedule of Available For Sale and Held to Maturity Securities [Line Items]    
Securities Available for Sale, Amortized Cost 280,758  
Securities Available for Sale, Gross Unrealized Gains 0  
Securities Available for Sale, Gross Unrealized (Losses) (2,143)  
Securities Available for Sale, Estimated Fair Value 278,615  
Securities Held to Maturity, Amortized Cost 0  
Securities Held to Maturity, Gross Unrealized Gains 0  
Securities Held to Maturity, Gross Unrealized (Losses) 0  
Securities Held to Maturity, Estimated Fair Value 0  
U.S. Government Agency Obligations [Member]    
Schedule of Available For Sale and Held to Maturity Securities [Line Items]    
Securities Available for Sale, Amortized Cost 15,253 18,378
Securities Available for Sale, Gross Unrealized Gains 88 144
Securities Available for Sale, Gross Unrealized (Losses) (362) (481)
Securities Available for Sale, Estimated Fair Value 14,979 18,041
Securities Held to Maturity, Amortized Cost 0 0
Securities Held to Maturity, Gross Unrealized Gains 0 0
Securities Held to Maturity, Gross Unrealized (Losses) 0 0
Securities Held to Maturity, Estimated Fair Value 0 0
Obligations of States and Political Subdivisions [Member]    
Schedule of Available For Sale and Held to Maturity Securities [Line Items]    
Securities Available for Sale, Amortized Cost 5,150 5,198
Securities Available for Sale, Gross Unrealized Gains 584 637
Securities Available for Sale, Gross Unrealized (Losses) 0 0
Securities Available for Sale, Estimated Fair Value 5,734 5,835
Securities Held to Maturity, Amortized Cost 10,683 26,584
Securities Held to Maturity, Gross Unrealized Gains 78 258
Securities Held to Maturity, Gross Unrealized (Losses) (4) (3)
Securities Held to Maturity, Estimated Fair Value 10,757 26,839
Residential Mortgage Pass-Through Securities Guaranteed by GNMA [Member]    
Schedule of Available For Sale and Held to Maturity Securities [Line Items]    
Securities Available for Sale, Amortized Cost 42,772 55,193
Securities Available for Sale, Gross Unrealized Gains 1,100 1,672
Securities Available for Sale, Gross Unrealized (Losses) (12) (3)
Securities Available for Sale, Estimated Fair Value 43,860 56,862
Securities Held to Maturity, Amortized Cost 5,912 7,598
Securities Held to Maturity, Gross Unrealized Gains 299 382
Securities Held to Maturity, Gross Unrealized (Losses) 0 0
Securities Held to Maturity, Estimated Fair Value 6,211 7,980
Residential Mortgage Pass-Through Securities Issued by FNMA and FHLMC [Member]    
Schedule of Available For Sale and Held to Maturity Securities [Line Items]    
Securities Available for Sale, Amortized Cost 2,185,140 1,421,861
Securities Available for Sale, Gross Unrealized Gains 13,366 20,768
Securities Available for Sale, Gross Unrealized (Losses) (11,094) (1,308)
Securities Available for Sale, Estimated Fair Value 2,187,412 1,441,321
Securities Held to Maturity, Amortized Cost 48,554 67,944
Securities Held to Maturity, Gross Unrealized Gains 1,468 2,397
Securities Held to Maturity, Gross Unrealized (Losses) 0 0
Securities Held to Maturity, Estimated Fair Value 50,022 70,341
Other Residential Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]    
Schedule of Available For Sale and Held to Maturity Securities [Line Items]    
Securities Available for Sale, Amortized Cost 231,793 409,883
Securities Available for Sale, Gross Unrealized Gains 5,146 9,600
Securities Available for Sale, Gross Unrealized (Losses) (54) (46)
Securities Available for Sale, Estimated Fair Value 236,885 419,437
Securities Held to Maturity, Amortized Cost 264,638 360,361
Securities Held to Maturity, Gross Unrealized Gains 13,143 19,678
Securities Held to Maturity, Gross Unrealized (Losses) (41) (55)
Securities Held to Maturity, Estimated Fair Value 277,740 379,984
Commercial Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]    
Schedule of Available For Sale and Held to Maturity Securities [Line Items]    
Securities Available for Sale, Amortized Cost 292,428 49,260
Securities Available for Sale, Gross Unrealized Gains 731 1,068
Securities Available for Sale, Gross Unrealized (Losses) (3,039) (9)
Securities Available for Sale, Estimated Fair Value 290,120 50,319
Securities Held to Maturity, Amortized Cost 65,118 75,585
Securities Held to Maturity, Gross Unrealized Gains 1,284 2,386
Securities Held to Maturity, Gross Unrealized (Losses) 0 0
Securities Held to Maturity, Estimated Fair Value $ 66,402 $ 77,971