Recent Developments Including Mergers and Acquisitions - Additional Information (Detail) - USD ($)			1 Months Ended
	Mar. 05, 2020	Aug. 15, 2019	Jan. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Business Combination Separately Recognized Transactions [Line Items]
Deposits				$ 8,495,891,000	$ 7,483,635,000
Goodwill				149,922,000	$ 148,604,000
Gain on sale of property held in other real estate owned			$ 2,200,000
BancFirst [Member]
Business Combination Separately Recognized Transactions [Line Items]
Sale of Loans				21,500,000
Deposits				38,000,000.0
COVID 19 [Member]
Business Combination Separately Recognized Transactions [Line Items]
Loans modification				$ 104,500,000
Citizens Bankshares, Inc [Member]
Business Combination Separately Recognized Transactions [Line Items]
Date of acquisition	Mar. 05, 2020
Total assets acquired	$ 47,800,000
Loans	22,900,000
Deposits	45,000,000.0
Purchase price	2,900,000
Intangibles assets, net	206,000
Goodwill	$ 1,300,000
Pegasus Bank [Member]
Business Combination Separately Recognized Transactions [Line Items]
Date of acquisition		Aug. 15, 2019