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Difference between Fair Value Option and Unpaid Principal Balance (Detail) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Fair Value, Option, Quantitative Disclosures [Line Items]    
Fair Value Carrying Amount $ 409,152 $ 367,221
Aggregate Unpaid Principal 408,928 359,587
Fair Value Carrying Amount Less Aggregate Unpaid Principal $ 224 $ 7,634