Quarterly Financial Results (Unaudited) (Detail) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 37,975,000	$ 37,150,000	$ 36,991,000	$ 36,250,000	$ 37,109,000	$ 40,180,000	$ 35,779,000	$ 36,279,000	$ 148,366,000	$ 149,347,000	$ 143,870,000
Interest expense	4,748,000	4,730,000	4,682,000	4,658,000	4,759,000	4,874,000	4,847,000	4,953,000	18,818,000	19,433,000	22,651,000
Net interest income	33,227,000	32,420,000	32,309,000	31,592,000	32,350,000	35,306,000	30,932,000	31,326,000	129,548,000	129,914,000	121,219,000
Provision (credit) for loan and lease losses	853,000	(192,000)	158,000	(982,000)	586,000	1,128,000	(2,876,000)	78,000	(163,000)	(1,084,000)	3,649,000
Non-interest income	11,338,000	12,590,000	11,694,000	11,249,000	11,654,000	11,223,000	12,215,000	12,419,000	46,871,000	47,511,000	46,956,000
Non-interest Expense	30,478,000	28,632,000	34,141,000	27,549,000	29,300,000	26,893,000	27,508,000	27,823,000	120,800,000	111,524,000	109,927,000
Income before income taxes	13,234,000	16,570,000	9,704,000	16,274,000	14,118,000	18,508,000	18,515,000	15,844,000	55,782,000	66,985,000	54,599,000
Income tax expense	4,086,000	5,428,000	2,722,000	5,346,000	4,505,000	6,419,000	6,353,000	5,286,000	17,582,000	22,563,000	18,045,000
Net income	$ 9,148,000	$ 11,142,000	$ 6,982,000	$ 10,928,000	$ 9,613,000	$ 12,089,000	$ 12,162,000	$ 10,558,000	$ 38,200,000	$ 44,422,000	$ 36,554,000
Earnings Per Share, Basic, Total	$ 0.37	$ 0.44	$ 0.28	$ 0.44	$ 0.38	$ 0.48	$ 0.49	$ 0.42	$ 1.53	$ 1.78	$ 1.49
Earnings Per Share, Diluted, Total	$ 0.36	$ 0.44	$ 0.28	$ 0.43	$ 0.38	$ 0.48	$ 0.49	$ 0.42	$ 1.52	$ 1.77	$ 1.48