Regulatory capital requirements - Additional Information (Detail)	Dec. 31, 2012
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Capital Required to be Well Capitalized to Risk Weighted Assets	10.00%
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets	6.00%
Tier One Leverage Capital Required to be Well Capitalized to Average Assets	5.00%

The Total risk based capital ratio (total capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Audit and Accounting Guide (AAG)

-Number AAG-DEP

-Paragraph 15

-Subparagraph 3a

-IssueDate 2006-05-01

-Chapter 17

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Audit and Accounting Guide (AAG)

-Number AAG-DEP

-Paragraph 22

-IssueDate 2006-05-01

-Chapter 17

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Name:	us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant