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INVESTMENT SECURITIES - Significant Assumptions in Valuation of Private Label MBS (Detail) - Private label MBS [Member]
Sep. 30, 2018
Dec. 31, 2017
Discount rate [Member] | Weighted Average [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Debt Securities, Available-for-sale, Measurement Input 0.148 0.14
Discount rate [Member] | Minimum [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Debt Securities, Available-for-sale, Measurement Input 0.148 0.14
Discount rate [Member] | Maximum [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Debt Securities, Available-for-sale, Measurement Input 0.148 0.14
Prepayment rate [Member] | Weighted Average [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Debt Securities, Available-for-sale, Measurement Input 0.117 0.164
Prepayment rate [Member] | Minimum [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Debt Securities, Available-for-sale, Measurement Input 0.035 0.12
Prepayment rate [Member] | Maximum [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Debt Securities, Available-for-sale, Measurement Input 0.225 0.29
Projected Cumulative Loss Rate [Member] | Weighted Average [Member]    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Debt Securities, Available-for-sale, Measurement Input 0.04 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 0.078 0.068