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INVESTMENT SECURITIES - Significant Assumptions in Valuation of Private Label MBS (Detail)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Schedule Of Investments [Line Items]    
Fair Value Inputs Discount Rate 14.00% 14.10%
Fair Value Inputs Prepayment Rate 16.40% 13.80%
Weighted Average, Projected Cumulative Loss Rate 3.00% 4.00%
Minimum [Member]    
Schedule Of Investments [Line Items]    
Fair Value Inputs Discount Rate   12.88%
Fair Value Inputs Prepayment Rate 12.00% 6.50%
Weighted Average, Projected Cumulative Loss Rate 0.00% 0.20%
Maximum [Member]    
Schedule Of Investments [Line Items]    
Fair Value Inputs Discount Rate   14.43%
Fair Value Inputs Prepayment Rate 29.00% 22.50%
Weighted Average, Projected Cumulative Loss Rate 6.80% 8.50%