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Intangible Assets and Goodwill - Additional Information (Detail) - USD ($)		12 Months Ended
	Aug. 15, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Weighted-average life		7 years 7 months 6 days
Goodwill		$ 148,604,000	$ 79,749,000	$ 54,042,000
Pegasus Bank [Member]
Finite-Lived Intangible Assets [Line Items]
Date of acquisition	Aug. 15, 2019
Goodwill	$ 68,900,000	$ 68,855,000
Customer Related Intangibles [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average life		4 years 9 months 18 days
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 9 months 18 days