INVESTMENT SECURITIES - Significant Assumptions in Valuation of Private Label MBS (Detail) - Private label MBS [Member]	Mar. 31, 2019	Dec. 31, 2018
Discount rate [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.141	0.145
Discount rate [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.141	0.145
Discount rate [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.141	0.145
Prepayment rate [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.092	0.114
Prepayment rate [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.033	0.033
Prepayment rate [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.17	0.209
Projected Cumulative Loss Rate [Member] | Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.03	0.03
Projected Cumulative Loss Rate [Member] | Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0	0
Projected Cumulative Loss Rate [Member] | Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Debt Securities, Available-for-sale, Measurement Input	0.066	0.068