Loans and reserve for credit losses (Post Modification Recorded in Investment for Troubled Debt Restructurings on Financing Receivables) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Rate reduction	$ 0	$ 0
Term extension	22	25
Rate reduction and term extension	0	0
Total	22	25
Commercial real estate portfolio
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Rate reduction	0	0
Term extension	0	0
Rate reduction and term extension	0	0
Total	0	0
Construction portfolio
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Rate reduction	0	0
Term extension	0	0
Rate reduction and term extension	0	0
Total	0	0
Residential real estate portfolio
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Rate reduction	0	0
Term extension	0	0
Rate reduction and term extension	0	0
Total	0	0
Commercial and industrial portfolio
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Rate reduction	0	0
Term extension	22	25
Rate reduction and term extension	0	0
Total	22	25
Consumer portfolio
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Rate reduction	0	0
Term extension	0	0
Rate reduction and term extension	0	0
Total	$ 0	$ 0