Loans and reserve for credit losses (Post Modification Recorded in Investment for Troubled Debt Restructurings on Financing Receivables) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Rate reduction	$ 3,983	$ 3,825
Term extension	4,674	7,426
Rate reduction and term extension	21,500	653
Total	30,157	11,904
Commercial Real Estate
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Rate reduction	3,809	1,299
Term extension	2,368	462
Rate reduction and term extension	21,500	0
Total	27,677	1,761
Construction
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Rate reduction	0	0
Term extension	1,243	4,797
Rate reduction and term extension	0	35
Total	1,243	4,832
Residential real estate
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Rate reduction		0
Term extension		579
Rate reduction and term extension		99
Total		678
Commercial and industrial loans
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Rate reduction	174	2,526
Term extension	1,063	965
Rate reduction and term extension	0	489
Total	1,237	3,980
Consumer Loans
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Rate reduction		0
Term extension		623
Rate reduction and term extension		30
Total		$ 653