Recorded Investment in Loans Modified in TDRs (Parenthetical) (Detail) (Commercial loans, USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2012
Commercial loans
Financing Receivable, Modifications [Line Items]
Loans classified as TDRs, charge-offs	$ 0.8

Reflects the amount of the write-down related to a troubled debt restructuring.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 40

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=6951948&loc=d3e18845-111554

Name:	us-gaap_FinancingReceivablesImpairedTroubledDebtRestructuringWriteDown
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration