Subsequent Event - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 22, 2021
Subsequent Event [Line Items]
Gain on transaction		$ 6,067	$ 3,619	$ 2,902
Total assets		9,212,357	8,565,758	$ 7,574,258
Total loans		6,394,506	5,662,143
Deposits		8,064,704	$ 7,483,635
First Bank & Trust Company [Member]
Subsequent Event [Line Items]
Total assets		285,000
Total loans		209,000
Deposits		$ 258,000
Scenario Forecast [Member]
Subsequent Event [Line Items]
Gain on transaction	$ 2,500
Subsequent Event [Member]
Subsequent Event [Line Items]
Sale of Loans					$ 21,000
Deposits sold					$ 38,000