Allowance for Loan Losses - Summary of Allowance for Loan Losses and Recorded Investments (Detail) - USD ($)		Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses, Individually Evaluated for Impairment		$ 1,260,000		$ 54,000
Allowance for Loan Losses, Collectively Evaluated for Impairment		48,647,000		47,857,000
Total Allowance for Loan Losses		49,907,000	$ 48,814,000	47,911,000	$ 47,316,000	$ 46,580,000	$ 46,255,000
Portfolio Loans, Individually Evaluated for Impairment		41,201,000		43,977,000
Portfolio Loans, Collectively Evaluated for Impairment		4,884,762,000		3,824,769,000
Total Portfolio Loans	[1]	4,925,963,000		3,868,746,000
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses, Individually Evaluated for Impairment		0
Allowance for Loan Losses, Collectively Evaluated for Impairment		16,978,000		20,164,000
Total Allowance for Loan Losses		16,978,000	19,018,000	20,164,000	19,977,000	20,733,000	18,921,000
Portfolio Loans, Individually Evaluated for Impairment		14,080,000		19,890,000
Portfolio Loans, Collectively Evaluated for Impairment		2,097,505,000		1,662,346,000
Total Portfolio Loans	[1]	2,111,585,000		1,682,236,000
Commercial Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses, Collectively Evaluated for Impairment		11,536,000		6,093,000
Total Allowance for Loan Losses		11,536,000	7,671,000	6,093,000	5,178,000	4,759,000	5,374,000
Portfolio Loans, Individually Evaluated for Impairment		9,315,000		7,605,000
Portfolio Loans, Collectively Evaluated for Impairment		375,013,000		208,543,000
Total Portfolio Loans	[1]	384,328,000		216,148,000
Consumer Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses, Individually Evaluated for Impairment		34,000		43,000
Allowance for Loan Losses, Collectively Evaluated for Impairment		6,982,000		6,290,000
Total Allowance for Loan Losses		7,016,000	7,027,000	6,333,000	6,763,000	6,705,000	6,362,000
Portfolio Loans, Individually Evaluated for Impairment		6,970,000		7,202,000
Portfolio Loans, Collectively Evaluated for Impairment		1,094,043,000		903,455,000
Total Portfolio Loans	[1]	1,101,013,000		910,657,000
Other Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses, Individually Evaluated for Impairment		2,000		11,000
Allowance for Loan Losses, Collectively Evaluated for Impairment		1,904,000		1,642,000
Total Allowance for Loan Losses		1,906,000	1,790,000	1,653,000	1,500,000	1,379,000	1,165,000
Portfolio Loans, Individually Evaluated for Impairment		121,000		62,000
Portfolio Loans, Collectively Evaluated for Impairment		91,001,000		65,505,000
Total Portfolio Loans	[1]	91,122,000		65,567,000
Fair Value Measurements [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses on acquired loans		0
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for Loan Losses, Individually Evaluated for Impairment		1,224,000
Allowance for Loan Losses, Collectively Evaluated for Impairment		11,247,000		13,668,000
Total Allowance for Loan Losses		12,471,000	$ 13,308,000	13,668,000	$ 13,898,000	$ 13,004,000	$ 14,433,000
Portfolio Loans, Individually Evaluated for Impairment		10,715,000		9,218,000
Portfolio Loans, Collectively Evaluated for Impairment		1,227,200,000		984,920,000
Total Portfolio Loans	[1]	$ 1,237,915,000		$ 994,138,000

[1] Includes acquired loans.