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Document and Entity Information - USD ($)
12 Months Ended
Dec. 31, 2017
Jan. 31, 2018
Jun. 30, 2017
Document And Entity Information [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2017    
Document Fiscal Year Focus 2017    
Document Fiscal Period Focus FY    
Trading Symbol BANF    
Entity Registrant Name BANCFIRST CORP /OK/    
Entity Central Index Key 0000760498    
Current Fiscal Year End Date --12-31    
Entity Well-known Seasoned Issuer Yes    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Entity Filer Category Large Accelerated Filer    
Entity Common Stock, Shares Outstanding   32,632,716  
Entity Public Float     $ 840,607,211