Securities - Additional Information (Detail)		12 Months Ended
	Apr. 01, 2014 USD ($) Security	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Investments, Debt and Equity Securities [Abstract]
Number of securities | Security	319
Aggregate fair value of securities	$ 167,100,000
Gain/Loss from net unrealized holdings, Net of tax	$ 1,300,000	$ 24,424,000	$ 16,693,000	$ (3,802,000)
Unrealized gain or loss, Held to maturity		0
Allowance for credit losses for available for sale debt securities		0
Accrued interest receivable on available for sale debt securities		8,100,000
Tax effect of the proceeds from sale of securities		902,000	$ (16,000)	$ (93,000)
Pledged of Fair Value of Securities as collateral		115,600,000
Amortization cost of securities as Collateral not Separately Reported		112,200,000
Debt Instrument Repurchase Agreement		109,500,000
Securities Pledged for Federal Home Loan Bank At Fair Value		505,900,000
Securities for Federal Home Loan Bank Not Separately Reported		471,200,000
Debt Instrument Federal Home Loan Bank		0
Securities Pledged For Derivative At Fair Value		49,700,000
Securities Pledged For Derivative At Amortized Cost		48,200,000
Debt Instrument Derivative Swap Agreement		$ 40,600,000