Investment Securities - Narrative (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2021 USD ($) Investment	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) Investment	Dec. 31, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
Carrying value of securities pledged to secure public deposits and other contractual obligations	$ 298,400		$ 249,600
Pledging requirements for credit derivatives and SWAP agreements - securities	$ 25,100		$ 32,600
Number of investments in non federal issuer representing more than 10% of shareholder's equity | Investment	0		0
Maximum investment in any single non-federal issuer representing shareholders equity	10.00%		10.00%
Fair value of securities HTM in unrealized loss position for which an ACL has not been recorded	$ 14,360		$ 0
Unrealized loss included in fair value of securities HTM in unrealized loss position for which an ACL has not been recorded	372		0
Fair value of securities AFS in unrealized loss position for which an ACL has been recorded	62,600
Unrealized loss position included in fair value of securities AFS in unrealized loss position for which an ACL has been recorded	1,400
Debt securities, available-for-sale, allowance for credit loss	(815)		(869)
Proceeds from sales	4,135	$ 65,621
Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Proceeds from sales	0
Corporate Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, allowance for credit loss	$ (815)	(692)	$ (869)	$ 0
Number of securities | Investment	14
Provision for credit losses, securities	$ 76
Available-for-sale Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Accrued interest receivable	593
Held-to-maturity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Accrued interest receivable	286
Other Noninterest Income [Member] | Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Net realized gain on investment securities	$ 164	$ (321)