Business Segments - Financial Information by Business Segment (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Interest and dividend income	$ 80,819	$ 70,092	$ 67,585	$ 67,601	$ 67,660	$ 66,935	$ 66,729	$ 60,379	$ 286,097	$ 261,712	$ 215,991
Interest expense	9,131	8,066	7,811	7,759	7,040	6,326	5,936	5,424	32,767	24,725	22,763
Net interest income	71,688	62,026	59,774	59,842	60,620	60,609	60,793	54,955	253,330	236,987	193,228
Provision for credit losses	2,128	2,214	1,811	2,324	2,585	1,798	2,681	1,289	8,478	8,353	6,405
Net interest income after provision for credit losses	69,560	59,812	57,963	57,518	58,035	58,811	58,112	53,666	244,852	228,634	186,823
Non-interest income									81,499	74,466	68,504
Non-interest expense	58,298	57,601	47,360	45,343	46,894	46,981	46,589	53,441	208,680	193,923	161,633
Income before provision for income taxes	32,682	23,228	30,194	31,568	31,167	30,016	29,595	18,415	117,671	109,177	93,694
Provision for income taxes	8,464	5,793	8,085	8,694	8,165	7,768	7,962	4,528	31,036	28,415	23,720
Net income	$ 24,218	$ 17,435	$ 22,109	$ 22,874	$ 23,002	$ 22,248	$ 21,633	$ 13,887	86,635	80,762	69,974
Community Banking [Member]
Segment Reporting Information [Line Items]
Interest and dividend income									286,097	261,712	215,991
Interest expense									32,767	24,725	22,763
Net interest income									253,330	236,987	193,228
Provision for credit losses									8,478	8,353	6,405
Net interest income after provision for credit losses									244,852	228,634	186,823
Non-interest income									59,869	52,566	47,435
Non-interest expense									196,784	181,821	149,429
Income before provision for income taxes									107,937	99,379	84,829
Provision for income taxes									27,142	24,496	20,174
Net income									80,795	74,883	64,655
Trust and Investment Services [Member]
Segment Reporting Information [Line Items]
Non-interest income									21,630	21,900	21,069
Non-interest expense									11,896	12,102	12,204
Income before provision for income taxes									9,734	9,798	8,865
Provision for income taxes									3,894	3,919	3,546
Net income									$ 5,840	$ 5,879	$ 5,319