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Investment Securities - Investment Securities in Continuous Unrealized Loss Position (Detail) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]    
Estimated fair value, Less than 12 months $ 35,727 $ 125,305
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses (551) (583)
Estimated fair value, 12 months or more 80,018 2,061,847
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses (8,519) (29,640)
Held to maturity, Estimated fair value, Less than 12 months 2,039 2,727
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses (11) (5)
Held to maturity, Estimated fair value, 12 months or more 52,418 194,891
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses (17,938) (20,168)
Total investment securities, fair value less than 12 months 37,766 128,032
Investment Securities Continuous Unrealized Loss Position Less Than Twelve Months Aggregate Losses (562) (588)
Total of investment securities, fair value, 12 Months or More 132,436 2,256,738
Investment Securities Continuous Unrealized Loss Position Twelve Months or Longer Aggregate Losses (26,457) (49,808)
U.S. Treasury and Federal Agencies [Member]    
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]    
Estimated fair value, Less than 12 months 985 1,406
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses (14) (7)
Estimated fair value, 12 months or more   2,893
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses   (9)
Government Issued or Guaranteed [Member]    
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]    
Estimated fair value, Less than 12 months 18,687 117,299
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses (356) (222)
Estimated fair value, 12 months or more 16,556 2,002,364
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses (399) (21,385)
Held to maturity, Estimated fair value, Less than 12 months 2,039 2,727
Held To Maturity Securities Continuous Unrealized Loss Position Less Than12 Months Aggregate Losses (11) (5)
Held to maturity, Estimated fair value, 12 months or more   145,235
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses   (1,987)
Other Debt Securities [Member]    
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]    
Estimated fair value, Less than 12 months 16,055 6,600
Available For Sale Securities Continuous Unrealized Losses Position Less Than Twelve Months Aggregate Losses (181) (354)
Estimated fair value, 12 months or more 63,462 56,313
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses (8,120) (8,243)
Obligations of States and Political Subdivisions [Member]    
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]    
Estimated fair value, 12 months or more   277
Available For Sale Securities Continuous Unrealized Loss Position Twelve Months Or More Aggregate Losses   (3)
Privately Issued [Member]    
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]    
Held to maturity, Estimated fair value, 12 months or more 52,418 49,656
Held To Maturity Securities Continuous Unrealized Loss Position12 Months Or Longer Aggregate Losses $ (17,938) $ (18,181)