Business Segment Reporting - Schedule of Segment Financial Results (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020		Jun. 30, 2019
Segment Reporting Information [Line Items]
Net interest income		$ 178,229	$ 166,414	$ 352,170		$ 330,282
Provision for credit losses	[1]	21,500	11,000	109,500	[2]	23,350	[2]
Noninterest income		120,456	105,398	218,880		212,780
Noninterest expense		208,533	193,387	397,152		384,013
Income before income taxes		68,652	67,425	64,398		135,699
Income tax expense (benefit)		8,123	10,466	7,308		20,996
NET INCOME		60,529	56,959	57,090		114,703
Average assets		28,136,000	23,270,000	27,088,000		23,063,000
Commercial Banking [Member]
Segment Reporting Information [Line Items]
Net interest income		113,085	102,579	220,034		202,393
Provision for credit losses		19,281	9,306	101,501		19,635
Noninterest income		24,078	20,387	35,318		43,568
Noninterest expense		62,123	68,511	121,166		135,331
Income before income taxes		55,759	45,149	32,685		90,995
Income tax expense (benefit)		6,597	7,008	3,709		14,079
NET INCOME		49,162	38,141	28,976		76,916
Average assets		12,390,000	10,668,000	12,000,000		10,558,000
Institutional Banking [Member]
Segment Reporting Information [Line Items]
Net interest income		28,129	30,501	61,164		62,252
Provision for credit losses		298	181	573		467
Noninterest income		66,488	55,626	128,440		111,476
Noninterest expense		76,953	64,852	145,406		130,249
Income before income taxes		17,366	21,094	43,625		43,012
Income tax expense (benefit)		2,055	3,274	4,951		6,655
NET INCOME		15,311	17,820	38,674		36,357
Average assets		9,671,000	7,249,000	9,340,000		7,132,000
Personal Banking [Member]
Segment Reporting Information [Line Items]
Net interest income		37,015	33,334	70,972		65,637
Provision for credit losses		1,921	1,513	7,426		3,248
Noninterest income		29,890	29,385	55,122		57,736
Noninterest expense		69,457	60,024	130,580		118,433
Income before income taxes		(4,473)	1,182	(11,912)		1,692
Income tax expense (benefit)		(529)	184	(1,352)		262
NET INCOME		(3,944)	998	(10,560)		1,430
Average assets		$ 6,075,000	$ 5,353,000	$ 5,748,000		$ 5,373,000

[1] For the three and six months ended June 30, 2019, this line represents the Provision for loan losses. See further discussion of this change in Note 3, “New Accounting Pronouncements.

[2] For the six months ended June 30, 2019, this line represents the Provision for loan losses. See further discussion of this change in Note 3, “New Accounting Pronouncements.