Business Combinations - Additional Information (Detail) (USD $)	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	May 15, 2014	Sep. 30, 2014 Branch
Business Acquisition [Line Items]
Business acquisition date	Oct. 16, 2009
Goodwill	$ 74,244,000	$ 55,097,000	$ 55,097,000
Non-recurring merger related expenses	1,973,000	0	0
American Security Bank [Member]
Business Acquisition [Line Items]
Business acquisition date	May 15, 2014
Acquired assets and assumed liabilities for cash				57,000,000
Number of bank branches				5
Goodwill				19,100,000
Total fair value of assets acquired				436,400,000
Cash and cash due from banks				117,800,000
Investment securities available for sale				44,500,000
FHLB stock				1,900,000
Loans receivable				242,700,000
Fixed assets				4,800,000
Core deposit intangible assets				2,100,000
Other real estate owned				1,600,000
Other assets				1,800,000
Total fair value of liabilities assumed				379,400,000
Deposits				378,400,000
Other liabilities				1,000,000
Non-recurring merger related expenses	$ 2,000,000
American Security Bank [Member] | Electronic Branch [Member]
Business Acquisition [Line Items]
Number of bank branches				2
Number of banking vestibules closed					2
American Security Bank [Member] | Loan Production Office [Member]
Business Acquisition [Line Items]
Number of bank branches				1