Business Segments - Financial Information by Business Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Interest and dividend income	$ 119,543	$ 98,888	$ 238,596	$ 185,203
Interest expense	21,083	16,541	41,774	29,667
Net interest income	98,460	82,347	196,822	155,536
Provision for credit losses	2,747	1,708	5,254	3,876
Net interest income after provision for credit losses	95,713	80,639	191,568	151,660
Non-interest income	31,156	23,408	58,929	47,491
Non-interest expense	71,952	63,543	146,385	118,114
Income before provision for income taxes	54,917	40,504	104,112	81,037
Provision for income taxes	10,103	7,335	18,961	14,339
Net income	44,814	33,169	85,151	66,698
Community Banking [Member]
Segment Reporting Information [Line Items]
Interest and dividend income	119,543	98,888	238,596	185,203
Interest expense	21,083	16,541	41,774	29,667
Net interest income	98,460	82,347	196,822	155,536
Provision for credit losses	2,747	1,708	5,254	3,876
Net interest income after provision for credit losses	95,713	80,639	191,568	151,660
Non-interest income	24,817	17,656	45,475	35,236
Non-interest expense	67,957	60,026	138,233	110,920
Income before provision for income taxes	52,573	38,269	98,810	75,976
Provision for income taxes	9,611	6,865	17,848	13,276
Net income	42,962	31,404	80,962	62,700
Trust and Investment Services [Member]
Segment Reporting Information [Line Items]
Non-interest income	6,339	5,752	13,454	12,255
Non-interest expense	3,995	3,517	8,152	7,194
Income before provision for income taxes	2,344	2,235	5,302	5,061
Provision for income taxes	492	470	1,113	1,063
Net income	$ 1,852	$ 1,765	$ 4,189	$ 3,998