Business Segments - Financial Information by Business Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Interest and dividend income	$ 98,888	$ 82,160	$ 185,203	$ 162,084
Interest expense	16,541	10,021	29,667	19,226
Net interest income	82,347	72,139	155,536	142,858
Provision for credit losses	1,708	2,383	3,876	5,094
Net interest income after provision for credit losses	80,639	69,756	151,660	137,764
Non-interest income	23,408	22,122	47,491	45,006
Non-interest expense	63,543	55,884	118,114	110,268
Income before provision for income taxes	40,504	35,994	81,037	72,502
Provision for income taxes	7,335	9,653	14,339	20,274
Net income	33,169	26,341	66,698	52,228
Community Banking [Member]
Segment Reporting Information [Line Items]
Interest and dividend income	98,888	82,160	185,203	162,084
Interest expense	16,541	10,021	29,667	19,226
Net interest income	82,347	72,139	155,536	142,858
Provision for credit losses	1,708	2,383	3,876	5,094
Net interest income after provision for credit losses	80,639	69,756	151,660	137,764
Non-interest income	17,656	16,550	35,236	33,290
Non-interest expense	60,026	52,754	110,920	103,746
Income before provision for income taxes	38,269	33,552	75,976	67,308
Provision for income taxes	6,865	8,676	13,276	18,196
Net income	31,404	24,876	62,700	49,112
Trust and Investment Services [Member]
Segment Reporting Information [Line Items]
Non-interest income	5,752	5,572	12,255	11,716
Non-interest expense	3,517	3,130	7,194	6,522
Income before provision for income taxes	2,235	2,442	5,061	5,194
Provision for income taxes	470	977	1,063	2,078
Net income	$ 1,765	$ 1,465	$ 3,998	$ 3,116