Investment Securities (Amortized Cost of held to Maturity Debt Securities by Year of Originations and Credit Rating) (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	$ 447,902	$ 450,965
Amortized cost	447,979	451,043
Agency Obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	8,998	7,600
Mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	50	62
State And Municipal Obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	428,631	433,655
Corporate Obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	10,300	9,726
AAA
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	66,255	65,492
AAA | Agency Obligations
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	8,998	7,600
AAA | Mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	50	62
AAA | State And Municipal Obligations
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	57,207	57,830
AAA | Corporate Obligations
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	0	0
AA
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	311,659	314,410
AA | Agency Obligations
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	0	0
AA | Mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	0	0
AA | State And Municipal Obligations
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	308,572	311,155
AA | Corporate Obligations
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	3,087	3,255
A
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	58,866	59,748
A | Agency Obligations
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	0	0
A | Mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	0	0
A | State And Municipal Obligations
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	51,678	53,302
A | Corporate Obligations
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	7,188	6,446
BBB
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	1,115	1,115
BBB | Agency Obligations
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	0	0
BBB | Mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	0	0
BBB | State And Municipal Obligations
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	1,115	1,115
BBB | Corporate Obligations
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	0	0
Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	10,084	10,278
Not Rated | Agency Obligations
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	0	0
Not Rated | Mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	0	0
Not Rated | State And Municipal Obligations
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	10,059	10,253
Not Rated | Corporate Obligations
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	$ 25	$ 25