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OTHER BORROWINGS - Components of Other Borrowings (Parenthetical) (Detail) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Sep. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Fbp Statutory Trust One [Member]      
Subordinated Borrowing [Line Items]      
Payments for Repurchase of Trust Preferred Securities   $ 23.8  
due June 17, 2034 [Member]      
Subordinated Borrowing [Line Items]      
Floating Interest rate on junior subordinated debentures 5.08%   4.35%
Subordinated Borrowing Due Date Jun. 17, 2034    
Callable step-rate notes rate 2.75%    
due June 17, 2034 [Member] | London Interbank Offered Rate (LIBOR) [Member]      
Subordinated Borrowing [Line Items]      
Debt Instrument Description Of Variable Rate Basis 3-month LIBOR   3-month LIBOR
due September 20, 2034 [Member]      
Subordinated Borrowing [Line Items]      
Floating Interest rate on junior subordinated debentures 4.84%   4.12%
Subordinated Borrowing Due Date Sep. 20, 2034    
Callable step-rate notes rate 2.50%    
due September 20, 2034 [Member] | London Interbank Offered Rate (LIBOR) [Member]      
Subordinated Borrowing [Line Items]      
Debt Instrument Description Of Variable Rate Basis 3-month LIBOR   3-month LIBOR
Junior Subordinated Debt [Member] | Fbp Statutory Trust One [Member]      
Subordinated Borrowing [Line Items]      
Payments for Repurchase of Trust Preferred Securities   $ 23.8