Securities Available-for-Sale - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) Security	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) Security	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) Security
Schedule of Available-for-sale Securities [Line Items]
Securities at fair value pledged to secure public deposits and repurchase agreements	$ 1,692,033,000		$ 1,692,033,000		$ 1,441,059,000
Securities available-for-sale are securities at fair value pledged for borrowing	32,590,000		32,590,000		$ 34,787,000
Gross gains on sales of securities	53,000	$ 105,000	61,000	$ 302,000
Proceeds from sales of securities available-for-sale			16,285,000	$ 27,517,000
Securities at floating rate or adjustable rate	$ 234,949,000		234,949,000
Federal Home Loan Bank [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross gains on sales of securities			$ 13,000
Securities AFS [Member]
Schedule of Available-for-sale Securities [Line Items]
Weighted average remaining life of investment securities available-for-sale			5 years 1 month 6 days
Number of securities, temporarily impaired for less than 12 months | Security	24		24		10
Number of securities, temporarily impaired for 12 months or longer | Security	18		18		30
Number of securities, temporarily impaired, total | Security	134		134		190
U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities at fair value pledged to secure public deposits and repurchase agreements	$ 196,356,000		$ 196,356,000		$ 197,304,000