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Difference between Fair Value Option and Unpaid Principal Balance (Detail) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Fair Value, Option, Quantitative Disclosures [Line Items]    
Fair Value Carrying Amount $ 215,981 $ 409,152
Aggregate Unpaid Principal 210,628 408,928
Fair Value Carrying Amount Less Aggregate Unpaid Principal $ 5,353 $ 224