Business Segments - Financial Information by Business Segment (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Interest and dividend income									$ 484,253	$ 414,957	$ 332,424
Interest expense									84,349	67,721	42,129
Net interest income									399,904	347,236	290,295
Provision for credit losses	$ 1,824	$ 4,121	$ 2,747	$ 2,507	$ 2,854	$ 1,035	$ 1,708	$ 2,168	11,198	7,764	9,986
Net interest income after provision for credit losses									388,706	339,472	280,309
Non-interest income									116,716	100,276	88,840
Non-interest expense	92,556	73,268	71,952	74,432	70,990	76,120	63,543	54,571	312,208	265,224	220,860
Income before provision for income taxes	43,422	45,681	54,917	49,195	54,484	39,002	40,504	40,533	193,214	174,524	148,289
Provision for income taxes	7,046	8,334	10,103	8,858	10,556	6,516	7,335	7,004	34,341	31,412	53,807
Net income	$ 36,376	$ 37,347	$ 44,814	$ 40,337	$ 43,928	$ 32,486	$ 33,169	$ 33,529	158,873	143,112	94,482
Community Banking [Member]
Segment Reporting Information [Line Items]
Interest and dividend income									484,253	414,957	332,424
Interest expense									84,349	67,721	42,129
Net interest income									399,904	347,236	290,295
Provision for credit losses									11,198	7,764	9,986
Net interest income after provision for credit losses									388,706	339,472	280,309
Non-interest income									90,137	75,653	66,100
Non-interest expense									295,747	250,338	207,441
Income before provision for income taxes									183,096	164,787	138,968
Provision for income taxes									32,216	29,367	50,079
Net income									150,880	135,420	88,889
Trust and Investment Services [Member]
Segment Reporting Information [Line Items]
Non-interest income									26,579	24,623	22,740
Non-interest expense									16,461	14,886	13,419
Income before provision for income taxes									10,118	9,737	9,321
Provision for income taxes									2,125	2,045	3,728
Net income									$ 7,993	$ 7,692	$ 5,593