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Nature of operations and basis of presentation (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Organization, Consolidation and Presentation of Financial Statements      
Reversal of accelerated FDIC Indemnification Asset, Accretion of Discount $ 0 $ 12,492 $ 0