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    
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    
Troubled Debt Restructuring, Debtor, Current Period [Line Items]    
Rate reduction   0
Term extension   623
Rate reduction and term extension   30
Total   $ 653