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Difference between Fair Value Option and Unpaid Principal Balance (Detail) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Fair Value, Option, Quantitative Disclosures [Line Items]    
Fair Value Carrying Amount $ 266,888 $ 409,152
Aggregate Unpaid Principal 260,464 408,928
Fair Value Carrying Amount Less Aggregate Unpaid Principal $ 6,424 $ 224