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INVESTMENT SECURITIES - Roll-Forward of Credit Losses on Debt Securities Held by Corporation (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]      
Credit losses at beginning of the period $ 28,981 $ 22,294 $ 5,777
Additions:      
Credit losses on debt securities for which an OTTI was no previously recognized 0 0 15,889
Credit losses on debt securities for which an OTTI was previously recognized 12,231 6,687 628
Reductions:      
Securities sold during the period (34,420)    
Ending balance of credit losses 6,792 28,981 22,294
Puerto Rico Government Obligations [Member]      
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]      
Credit losses at beginning of the period 22,189 15,889 0
Additions:      
Credit losses on debt securities for which an OTTI was no previously recognized 0 0 15,889
Credit losses on debt securities for which an OTTI was previously recognized 12,231 6,300 0
Reductions:      
Securities sold during the period (34,420)    
Ending balance of credit losses 0 22,189 15,889
Mortgage Backed Securities Issued By Private Enterprises [Member]      
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items]      
Credit losses at beginning of the period 6,792 6,405 5,777
Additions:      
Credit losses on debt securities for which an OTTI was no previously recognized 0 0 0
Credit losses on debt securities for which an OTTI was previously recognized 0 387 628
Reductions:      
Securities sold during the period 0    
Ending balance of credit losses $ 6,792 $ 6,792 $ 6,405