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Investment Securities (Amortized Cost of held to Maturity Debt Securities by Year of Originations and Credit Rating) (Detail) - USD ($)
$ in Thousands
Sep. 30, 2020
Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost $ 446,591 $ 453,629
Total 446,667 453,629
Agency Obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Total 9,100 6,599
Mortgage-Backed Securities    
Schedule of Held-to-maturity Securities [Line Items]    
Total 75 118
State And Municipal Obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Total 428,532 437,074
Corporate Obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Total 8,960 9,838
AAA    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 59,303 56,033
AAA | Agency Obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 9,100 6,599
AAA | Mortgage-Backed Securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 75 118
AAA | State And Municipal Obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 50,128 49,316
AAA | Corporate Obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 0 0
AA    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 315,772 333,450
AA | Agency Obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 0 0
AA | Mortgage-Backed Securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 0 0
AA | State And Municipal Obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 313,053 330,322
AA | Corporate Obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 2,719 3,128
A    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 60,033 62,652
A | Agency Obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 0 0
A | Mortgage-Backed Securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 0 0
A | State And Municipal Obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 54,217 56,317
A | Corporate Obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 5,816 6,335
BBB    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 1,515 1,469
BBB | Agency Obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 0 0
BBB | Mortgage-Backed Securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 0 0
BBB | State And Municipal Obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 1,115 1,119
BBB | Corporate Obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 400 350
Not Rated    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 10,044 25
Not Rated | Agency Obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 0 0
Not Rated | Mortgage-Backed Securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 0 0
Not Rated | State And Municipal Obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 10,019 0
Not Rated | Corporate Obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost $ 25 $ 25