Allowance for Loan and Lease Losses ("ALLL") and Credit Quality Indicators - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Financing Receivable, Recorded Investment, Past Due [Line Items]
Partial charge-offs on purchased loans	$ 400,000	$ 1,100,000	$ 1,700,000	$ 1,300,000
Provision for purchased loans charged off	400,000	1,100,000	1,700,000	1,300,000
Impaired purchased loans	12,347,000	21,205,000	12,347,000	21,205,000		$ 14,040,000
Allowance for Loan and Lease Losses	56,749,000	46,958,000	56,749,000	46,958,000	$ 54,147,000	52,918,000	$ 43,861,000	$ 42,945,000
Non-Purchased Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Allowance for Loan and Lease Losses	$ 0	$ 0	$ 0	$ 0		$ 0