Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
Sep. 30, 2012	Dec. 31, 2011
Schedule of Trading Securities and Other Trading Assets [Line Items]
Gross unrealized losses related to available for sale securities	Less than $1 thousand	Less than $1 thousand
Gross unrealized losses	$ (29,645)
Maximum [Member] \| Mortgage Backed Securities - Residential [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Gross unrealized losses	$ 1	1

The net amount of gross unrealized gains (losses) for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27357-111563

Name:	us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGainLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant