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Intangible Assets and Goodwill - Additional Information (Detail) - USD ($)			12 Months Ended
	Mar. 05, 2020	Aug. 15, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Weighted-average life			7 years 2 months 12 days
Goodwill			$ 149,922,000	$ 148,604,000	$ 79,749,000
Pegasus Bank [Member]
Finite-Lived Intangible Assets [Line Items]
Date of acquisition		Aug. 15, 2019
Goodwill		$ 68,900,000	$ 68,855,000	68,855,000
Citizens Bankshares, Inc [Member]
Finite-Lived Intangible Assets [Line Items]
Date of acquisition	Mar. 05, 2020
Goodwill	$ 1,300,000
Customer Related Intangibles [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average life			4 years
Customer Related Intangibles [Member] \| BancFirst Insurance Services, Inc. [Member]
Finite-Lived Intangible Assets [Line Items]
Impairment of intangible assets				$ 353,000
Core Deposit Intangibles [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average life			7 years 3 months 18 days