FAIR VLUE - Assets and Liabilites Measured at Fair Value on Recurring Basis Using Unobservable Inputs (Level 3) (Detail) $ in Thousands	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Debt Securities, Available-for-sale, Sale [Abstract]
Debt Securities, Available-for-sale	$ 1,905,230	$ 1,942,568
Minimum [Member] | Private label MBS [Member] | Discount rate [Member]
Debt Securities, Available-for-sale, Sale [Abstract]
Debt Securities, Available-for-sale, Measurement Input	0.141	0.145
Minimum [Member] | Private label MBS [Member] | Projected Cumulative Loss Rate [Member]
Debt Securities, Available-for-sale, Sale [Abstract]
Debt Securities, Available-for-sale, Measurement Input	0	0
Maximum [Member] | Private label MBS [Member] | Discount rate [Member]
Debt Securities, Available-for-sale, Sale [Abstract]
Debt Securities, Available-for-sale, Measurement Input	0.141	0.145
Maximum [Member] | Private label MBS [Member] | Projected Cumulative Loss Rate [Member]
Debt Securities, Available-for-sale, Sale [Abstract]
Debt Securities, Available-for-sale, Measurement Input	0.066	0.068
Weighted Average [Member] | Private label MBS [Member] | Discount rate [Member]
Debt Securities, Available-for-sale, Sale [Abstract]
Debt Securities, Available-for-sale, Measurement Input	0.141	0.145
Weighted Average [Member] | Private label MBS [Member] | Projected Cumulative Loss Rate [Member]
Debt Securities, Available-for-sale, Sale [Abstract]
Debt Securities, Available-for-sale, Measurement Input	0.03	0.03
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Private label MBS [Member] | Discounted Cash Flow [Member]
Debt Securities, Available-for-sale, Sale [Abstract]
Debt Securities, Available-for-sale	$ 13,351	$ 13,914
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Private label MBS [Member] | Discount rate [Member] | Discounted Cash Flow [Member]
Debt Securities, Available-for-sale, Sale [Abstract]
Debt Securities, Available-for-sale, Measurement Input	0.141	0.145
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Puerto Rico Government obligations [Member] | Discounted Cash Flow [Member]
Debt Securities, Available-for-sale, Sale [Abstract]
Debt Securities, Available-for-sale	$ 2,855	$ 2,824
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Puerto Rico Government obligations [Member] | Discount rate [Member] | Discounted Cash Flow [Member]
Debt Securities, Available-for-sale, Sale [Abstract]
Debt Securities, Available-for-sale, Measurement Input	0.0656	0.0628
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Puerto Rico Government obligations [Member] | Prepayment rate [Member] | Discounted Cash Flow [Member]
Debt Securities, Available-for-sale, Sale [Abstract]
Debt Securities, Available-for-sale, Measurement Input	0.03	0.03
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Minimum [Member] | Private label MBS [Member] | Prepayment rate [Member] | Discounted Cash Flow [Member]
Debt Securities, Available-for-sale, Sale [Abstract]
Debt Securities, Available-for-sale, Measurement Input	0.033	0.033
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Minimum [Member] | Private label MBS [Member] | Projected Cumulative Loss Rate [Member] | Discounted Cash Flow [Member]
Debt Securities, Available-for-sale, Sale [Abstract]
Debt Securities, Available-for-sale, Measurement Input	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Maximum [Member] | Private label MBS [Member] | Prepayment rate [Member] | Discounted Cash Flow [Member]
Debt Securities, Available-for-sale, Sale [Abstract]
Debt Securities, Available-for-sale, Measurement Input	0.17	0.209
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Maximum [Member] | Private label MBS [Member] | Projected Cumulative Loss Rate [Member] | Discounted Cash Flow [Member]
Debt Securities, Available-for-sale, Sale [Abstract]
Debt Securities, Available-for-sale, Measurement Input	0.066	0.068
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Weighted Average [Member] | Private label MBS [Member] | Prepayment rate [Member] | Discounted Cash Flow [Member]
Debt Securities, Available-for-sale, Sale [Abstract]
Debt Securities, Available-for-sale, Measurement Input	0.092	0.114
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Weighted Average [Member] | Private label MBS [Member] | Projected Cumulative Loss Rate [Member] | Discounted Cash Flow [Member]
Debt Securities, Available-for-sale, Sale [Abstract]
Debt Securities, Available-for-sale, Measurement Input	0.03	0.03