Investment Securities (Amortized Cost of held to Maturity Debt Securities by Year of Originations and Credit Rating) (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 427,081	$ 451,043
Agency obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	9,996	7,600
Mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	29	62
State and municipal obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	407,424	433,655
Corporate obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	9,632	9,726
AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	58,373	65,492
AAA | Agency obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	9,996	7,600
AAA | Mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	29	62
AAA | State and municipal obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	48,348	57,830
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	312,329	314,410
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	309,708	311,155
AA | Corporate obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	2,621	3,255
A
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	55,043	59,748
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	48,057	53,302
A | Corporate obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	6,986	6,446
BBB
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	655	1,115
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	655	1,115
BBB | Corporate obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	0	0
Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	681	10,278
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	656	10,253
Not Rated | Corporate obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 25	$ 25