Fair Value Measurements - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Transfer of securities between levels of fair value hierarchy		$ 0
Days from origination after which mortgage loans are sold	30 days
Amount of non-financial assets (liabilities) measured at fair value on a recurring basis	$ 0	0
Non-financial assets or liabilities for which no impairment was provided	0	0
BancFirst [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Loans held for sale	21,500,000
Level 2 Inputs [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Loans held for sale	48,601,000	$ 11,001,000
Level 2 Inputs [Member] | BancFirst [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Loans held for sale	$ 21,500,000