Business Segment Reporting - Schedule of Segment Financial Results (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Net interest income		$ 194,675	$ 184,384	$ 178,229	$ 173,941	$ 172,363	$ 168,260	$ 166,414	$ 163,868	$ 731,229	$ 670,905	$ 610,446
Provision for credit losses	[1]									130,500	32,850	70,750
Noninterest income		228,290	112,996	120,456	98,424	110,355	103,635	105,398	107,382	560,166	426,770	401,698
Noninterest expense		226,858	197,995	208,533	188,619	203,450	191,397	193,387	190,626	822,005	778,860	717,800
Income before income taxes										338,890	285,965	223,594
Income tax expense (benefit)		34,787	10,293	8,123	(815)	10,753	10,616	10,466	10,530	52,388	42,365	27,334
Income from continuing operations		$ 156,320	$ 73,092	$ 60,529	$ (3,439)	$ 66,515	$ 62,382	$ 56,959	$ 57,744	286,502	243,600	196,260
Average assets										28,568,000	23,785,000	21,000,000
Commercial Banking [Member]
Segment Reporting Information [Line Items]
Net interest income										475,425	412,232	380,266
Provision for credit losses										119,424	26,159	63,841
Noninterest income										189,412	81,609	74,931
Noninterest expense										272,283	267,345	253,740
Income before income taxes										273,130	200,337	137,616
Income tax expense (benefit)										42,223	29,679	16,824
Income from continuing operations										230,907	170,658	120,792
Average assets										12,614,000	10,740,000	9,858,000
Institutional Banking [Member]
Segment Reporting Information [Line Items]
Net interest income										106,856	126,591	105,135
Provision for credit losses										882	975	1,335
Noninterest income										254,874	232,444	208,423
Noninterest expense										286,635	268,423	238,654
Income before income taxes										74,213	89,637	73,569
Income tax expense (benefit)										11,472	13,280	8,993
Income from continuing operations										62,741	76,357	64,576
Average assets										9,746,000	7,714,000	6,184,000
Personal Banking [Member]
Segment Reporting Information [Line Items]
Net interest income										148,948	132,082	125,045
Provision for credit losses										10,194	5,716	5,574
Noninterest income										115,880	112,717	118,344
Noninterest expense										263,087	243,092	225,406
Income before income taxes										(8,453)	(4,009)	12,409
Income tax expense (benefit)										(1,307)	(594)	1,517
Income from continuing operations										(7,146)	(3,415)	10,892
Average assets										$ 6,208,000	$ 5,331,000	$ 4,958,000

[1] For the years ended December 31, 2019 and 2018, this line represents the Provision for loan losses. See further discussion of this change in Note 2, “New Accounting Pronouncements.”