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Difference between Fair Value Option and Unpaid Principal Balance (Detail) - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
Fair Value, Option, Quantitative Disclosures [Line Items]    
Fair Value Carrying Amount $ 367,221 $ 201,012
Aggregate Unpaid Principal 359,587 194,692
Fair Value Carrying Amount Less Aggregate Unpaid Principal $ 7,634 $ 6,320