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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details)
$ / shares in Units, $ in Millions
 3 Months Ended
Oct. 18, 2021
USD ($)
office
branch
$ / shares
shares
Jun. 30, 2022
USD ($)
branch
Restructuring Cost and Reserve [Line Items]    
Number of banking offices | branch   31
Purchase of mortgage servicing rights   $ 22.8
Unpaid principal balance on underlying loans of MSRs   $ 1,730.0
Common Stock    
Restructuring Cost and Reserve [Line Items]    
Closing price of BOC's common stock (in USD per share) | $ / shares $ 18.74  
Pacific Mercantile Bancorp (“PMB”)    
Restructuring Cost and Reserve [Line Items]    
Exchange ratio 0.5  
Pacific Mercantile Bancorp (“PMB”)    
Restructuring Cost and Reserve [Line Items]    
Number of banking offices | office 7  
Number of full service brances | branch 3  
Acquisitions shares (in shares) | shares 23,713,417  
Cash paid at closing $ 3.2  
Shares paid as merger consideration (in shares) | shares 11,856,713  
Stock consideration $ 222.2  
Aggregate purchase price 225.4  
Goodwill $ 58.0