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Note Borrowings (Additional Information) (Detail) - USD ($)
3 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Debt Disclosure [Line Items]    
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Maximum Amount Available $ 3,400,000,000 $ 3,900,000,000
Federal Home Loan Bank Advances And Letters Of Credit Amount Used 600,000,000 726,000,000
BanPonce Trust I    
Debt Disclosure [Line Items]    
Subordinated Debt $ 55,000,000  
Due 2019 | Senior notes    
Debt Disclosure [Line Items]    
Interest Rate Percentage 7.00%  
Senior Notes $ 450,000,000  
Debt instrument maturity date Oct. 15, 2018  
Debt instrument maturity year 2019  
Due 2023 | Senior notes    
Debt Disclosure [Line Items]    
Principal amount $ 300,000,000  
Interest Rate Percentage 6.125%  
Debt issuance costs $ 6,300,000  
Debt instrument maturity year 2023  
Federal Reserve Bank Advances    
Debt Disclosure [Line Items]    
Line Of Credit Facility Current Borrowing Capacity $ 1,200,000,000 1,100,000,000
Line Of Credit Facility Capacity Available For Specific Purpose Other Than For Trade Purchases $ 1,100,000,000 $ 260,000,000