Business Combinations and Asset Acquisitions (Detail) $ in Thousands				9 Months Ended
	Nov. 01, 2019 USD ($) locations	Oct. 07, 2019 USD ($) locations	May 24, 2019 USD ($) locations	Sep. 30, 2020 USD ($)
Business Acquisition [Line Items]
Goodwill recorded on acquisition				$ 0
SBC
Business Acquisition [Line Items]
Number of locations acquired | locations	6
Business combination, assets acquired	$ 619,800
Business combination, loans acquired	423,000
Business combination, deposits acquired	507,800
Goodwill recorded on acquisition	$ 40,300
STC
Business Acquisition [Line Items]
Number of locations acquired | locations		5
Business combination, assets acquired		$ 250,100
Business combination, loans acquired		174,300
Business combination, deposits acquired		201,200
Goodwill recorded on acquisition		$ 19,100
ROC
Business Acquisition [Line Items]
Number of locations acquired | locations			1
Business combination, assets acquired			$ 223,400
Business combination, loans acquired			124,700
Business combination, deposits acquired			161,200
Goodwill recorded on acquisition			$ 11,700