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FEDERAL HOME LOAN BANK ADVANCES AND OTHER BORROWINGS - Additional Information (Detail) - USD ($)
 6 Months Ended
Jun. 30, 2017
Jun. 30, 2017
Mar. 31, 2018
Dec. 31, 2017
Line of Credit Facility [Line Items]        
Advances from FHLB collateralized     $ 3,480,000,000 $ 2,900,000,000
Securities sold under repurchase agreements     0 0
Line of Credit | Unsecured Federal Funds Line of Credit        
Line of Credit Facility [Line Items]        
Line of credit facility     210,000,000  
Line of Credit | Line Of Credit Facility Maturing July 17, 2017        
Line of Credit Facility [Line Items]        
Line of credit facility       75,000,000
Repayment of line of credit $ 50,000,000      
Line of Credit | Line Of Credit Facility Maturing July 17, 2017 | London Interbank Offered Rate (LIBOR)        
Line of Credit Facility [Line Items]        
Basis spread on variable rate   2.25%    
Federal Reserve Bank Advances        
Line of Credit Facility [Line Items]        
Line of credit facility     64,700,000  
Other borrowings     0  
Securities        
Line of Credit Facility [Line Items]        
Advances from FHLB collateralized     422,200,000 405,600,000
Securities | Federal Reserve Bank Advances        
Line of Credit Facility [Line Items]        
Collateral on line of credit facility     108,100,000  
Federal Home Loan Bank of San Francisco        
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]        
FHLB stock     55,800,000 $ 48,700,000
Amount of additional available borrowing     $ 1,090,000,000