Stockholders' Equity - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Jan. 01, 2021	Dec. 31, 2019
Stockholders Equity [Line Items]
Increase to retained earnings for cumulative effect of adopting ASU	$ 871,161,000		$ 815,488,000
Small Business Administration (SBA), CARES Act, Paycheck Protection Program Lending Facility [Member]
Stockholders Equity [Line Items]
Percentage of risk weight under risk based capital rules	0.00%
Cumulative Effect Period of Adoption Adjustment [Member]
Stockholders Equity [Line Items]
Increase to retained earnings for cumulative effect of adopting ASU	$ 2,300,000
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Maximum [Member]
Stockholders Equity [Line Items]
Quantitative limit for trust preferred securities to be included in tier 1 capital	$ 15,000,000,000
Subsequent Event [Member]
Stockholders Equity [Line Items]
Available equity of BancFirst for dividend payments to the company		$ 102,100,000
Senior Preferred Stock [Member]
Stockholders Equity [Line Items]
Preferred stock, shares issued	0		0
Preferred stock, shares outstanding	0
Cumulative Preferred Stock [Member]
Stockholders Equity [Line Items]
Preferred stock, shares issued	0		0
Preferred stock, shares outstanding	0
Dividend payable at redemption, per share	$ 5.00
Cumulative preferred stock dividend rate	10.00%