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Merger accounting - Summary of Acquired Nonimpaired and Impaired Loans (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014
Acquired Non Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractually required principal and interest at acquisition	$ 501,423
Expected cash flows at acquisition	501,423
Interest component of expected cash flows (accretable discount)	(4,773)
Fair value of acquired loans	496,650
Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractually required principal and interest at acquisition	19,353
Contractual cash flows not expected to be collected (nonaccretable discount)	(10,873)
Expected cash flows at acquisition	8,480
Interest component of expected cash flows (accretable discount)	(1,128)
Fair value of acquired loans	$ 7,352