Investment Securities - Additional Information (Detail) (USD $)	3 Months Ended	9 Months Ended
Sep. 30, 2012	Sep. 30, 2012	Sep. 30, 2013 Investment	Dec. 31, 2012 Investment
Investments Debt And Equity Securities [Abstract]
Carrying value of securities pledged to secure public deposits and for other purposes as required by law			$ 330,800,000	$ 368,200,000
Realized other-than-temporary impairment charges to noninterest income on equity portfolio	$ 4,000	$ 13,000
Number of investments in non federal issuer representing more than 10% of shareholders' equity			0	0
Maximum investment in any single non-federal issuer representing shareholders' equity			10.00%	10.00%

The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 35

-Paragraph 34

-URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e25351-111560

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