Bank Premises and Equipment - Additional Information (Detail) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property, Plant and Equipment [Abstract]
Lease expense	$ 2,465,000us-gaap_LeaseAndRentalExpense	$ 2,094,000us-gaap_LeaseAndRentalExpense	$ 2,055,000us-gaap_LeaseAndRentalExpense
Lease rental income	$ 307,000	$ 299,000	$ 329,000
Operating lease premises and equipment lease expiration year, maximum	2026

The total amount of sublease rental income recognized during the period that reduces the entity's rent expense incurred under operating leases.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 10

-Section 55

-Paragraph 40

-Subparagraph (Note 3)

-URI http://asc.fasb.org/extlink&oid=6584154&loc=d3e38371-112697

Name:	us-gaap_OperatingLeasesRentExpenseSubleaseRentals1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration