Loans - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2013
Accounts Notes And Loans Receivable [Line Items]
Evaluation period of net charge-off history			2 years 9 months		2 years
Recorded investment in trouble debt restructuring, valuation allowance	$ 16,881,000	$ 18,264,000	$ 16,881,000	$ 18,264,000	$ 17,687,000	$ 17,221,000	$ 20,554,000	$ 21,116,000
Amounts charged off	1,464,000	1,192,000	$ 2,176,000	2,356,000
Period of cumulative homogeneous loans past due included in company analysis			90 days
Troubled Debt Restructurings
Accounts Notes And Loans Receivable [Line Items]
Recorded investment in trouble debt restructuring	29,500,000		$ 29,500,000		28,100,000
Recorded investment in trouble debt restructuring, valuation allowance	2,400,000		2,400,000		2,600,000
commitment to lend additional amounts	41,000		41,000		$ 50,000
Increment in allowance for loan losses	24,000	59,000	82,000	116,000
Amounts charged off	0	3,000	0	$ 3,000
TDRs with Subsequent Default [Member]
Accounts Notes And Loans Receivable [Line Items]
Amounts charged off	0	0	0
Effect of troubled debt restructurings on provision for loan losses	$ 0	$ 0	$ 0