Significant accounting policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2012
Significant Accounting Policies [Line Items]
Delinquent period in which principal or interest is charged against income (in days)	90 days
Delinquent period in which the net realizable value of the property collateralizing loan is charged-off	150 days
Consumer loans delay in payments	90 days or less
Consumer loans due in days	91 to 180
Estimated useful lives of core deposit and other intangible assets	8 years
Minimum [Member]
Significant Accounting Policies [Line Items]
Estimated useful lives of core deposit and other intangible assets	5 years
Income tax benefit recognition criteria percentage threshold	50.00%
Maximum [Member]
Significant Accounting Policies [Line Items]
Estimated useful lives of core deposit and other intangible assets	10 years