Allowance for Loan Losses - Summary of Allowance for Loan Losses and Recorded Investments (Detail) - USD ($)	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses, Individually Evaluated for Impairment	$ 608,000		$ 54,000
Allowance for Loan Losses, Collectively Evaluated for Impairment	48,206,000		47,857,000
Total Allowance for Loan Losses	48,814,000	$ 48,106,000	47,911,000	$ 46,580,000	$ 46,616,000	$ 46,255,000
Portfolio Loans, Individually Evaluated for Impairment	45,855,000		43,977,000
Portfolio Loans, Collectively Evaluated for Impairment	4,752,643,000		3,824,769,000
Total Portfolio Loans	4,798,498,000		3,868,746,000
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses, Individually Evaluated for Impairment	567,000
Allowance for Loan Losses, Collectively Evaluated for Impairment	18,451,000		20,164,000
Total Allowance for Loan Losses	19,018,000	19,071,000	20,164,000	20,733,000	19,880,000	18,921,000
Portfolio Loans, Individually Evaluated for Impairment	19,297,000		19,890,000
Portfolio Loans, Collectively Evaluated for Impairment	2,035,638,000		1,662,346,000
Total Portfolio Loans	2,054,935,000		1,682,236,000
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses, Collectively Evaluated for Impairment	13,308,000		13,668,000
Total Allowance for Loan Losses	13,308,000	13,711,000	13,668,000	13,004,000	13,979,000	14,433,000
Portfolio Loans, Individually Evaluated for Impairment	10,125,000		9,218,000
Portfolio Loans, Collectively Evaluated for Impairment	1,229,257,000		984,920,000
Total Portfolio Loans	1,239,382,000		994,138,000
Construction Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses, Collectively Evaluated for Impairment	7,671,000		6,093,000
Total Allowance for Loan Losses	7,671,000	6,869,000	6,093,000	4,759,000	5,183,000	5,374,000
Portfolio Loans, Individually Evaluated for Impairment	9,293,000		7,605,000
Portfolio Loans, Collectively Evaluated for Impairment	335,690,000		208,543,000
Total Portfolio Loans	344,983,000		216,148,000
Consumer Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses, Individually Evaluated for Impairment	37,000		43,000
Allowance for Loan Losses, Collectively Evaluated for Impairment	6,990,000		6,290,000
Total Allowance for Loan Losses	7,027,000	6,723,000	6,333,000	6,705,000	6,408,000	6,362,000
Portfolio Loans, Individually Evaluated for Impairment	6,944,000		7,202,000
Portfolio Loans, Collectively Evaluated for Impairment	1,057,817,000		903,455,000
Total Portfolio Loans	1,064,761,000		910,657,000
Other Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses, Individually Evaluated for Impairment	4,000		11,000
Allowance for Loan Losses, Collectively Evaluated for Impairment	1,786,000		1,642,000
Total Allowance for Loan Losses	1,790,000	$ 1,732,000	1,653,000	$ 1,379,000	$ 1,166,000	$ 1,165,000
Portfolio Loans, Individually Evaluated for Impairment	196,000		62,000
Portfolio Loans, Collectively Evaluated for Impairment	94,241,000		65,505,000
Total Portfolio Loans	94,437,000		$ 65,567,000
Fair Value Measurements [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses on acquired loans	$ 0