Fair Value Disclosures of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	$ 756,731,000	$ 756,731,000	$ 740,124,000
Minimum discount rate on appraisals of collateral securing loan		10.00%
Maximum discount rate on appraisals of collateral securing loan		50.00%
Collateral for collateral dependent loans	37,056,000	$ 37,056,000	41,334,000
Valuation allowance on impaired loans	3,000,000	3,000,000	5,300,000
Commitments to Extend Credit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commitments of lending operations	0		0
Impaired Loans (Collateral Dependent) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Collateral for collateral dependent loans	40,100,000	40,100,000	$ 46,700,000
Federal Agency Debentures [Member] | AAA-Rated [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	31,100,000	31,100,000
Federal Agency MBS [Member] | AAA-Rated [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	$ 725,600,000	$ 725,600,000
Visa [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Conversion rate of Class A stock for each share of Class B stock		1.6483%
Visa [Member] | Common Class B [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Shares held by company	50,833	50,833
Estimated value of shares	$ 5,800,000	$ 5,800,000