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Held to Maturity Debt Securities - Amortized Cost of held to Maturity Debt Securities by Year of Originations and Credit Rating (Detail) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost $ 450,965 $ 453,629
Total 451,043 453,629
Agency obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Total 7,600 6,599
Mortgage-backed securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 62  
Total 62 118
State and municipal obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Total 433,655 437,074
Corporate obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Total 9,726 9,838
AAA    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 65,492 56,033
AAA | Agency obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 7,600 6,599
AAA | Mortgage-backed securities    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 62 118
AAA | State and municipal obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 57,830 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 314,410 333,450
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 311,155 330,322
AA | Corporate obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 3,255 3,128
A    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 59,748 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 53,302 56,317
A | Corporate obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 6,446 6,335
BBB    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 1,115 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 0 350
Not Rated    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost 10,278 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 10,253 0
Not Rated | Corporate obligations    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost $ 25 $ 25