Segment Information (Summary of Segment Information) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Net interest income:
Net interest income	$ 264,852	$ 247,563	$ 793,040	$ 710,815
Net interest income, Change in Contribution	$ 17,289		$ 82,225
Net interest income, Change in Contribution (as a percent)	7.00%		12.00%
Non-interest income:
Non-interest income	$ 115,137	99,930	$ 294,952	280,842
Non-interest income, Change in Contribution	$ 15,207		$ 14,110
Non-interest income, Change in Contribution (as a percent)	15.00%		5.00%
Net revenue:
Net revenue	$ 379,989	347,493	$ 1,087,992	991,657
Net revenue, Change in Contribution	$ 32,496		$ 96,335
Net revenue, Change in Contribution (as a percent)	9.00%		10.00%
Segment profit:
Segment profit	$ 99,121	91,948	$ 269,733	263,509
Segment profit, Change in Contribution	$ 7,173		$ 6,224
Segment profit, Change in Contribution (as a percent)	8.00%		2.00%
Segment assets:
Segment assets	$ 34,911,902	30,142,731	$ 34,911,902	30,142,731	$ 31,244,849
Segment assets, Change in Contribution	$ 4,769,171
Segment assets, Change in Contribution (as a percent)	16.00%
Community banking
Segment profit:
Segment profit	$ 68,792	63,735	182,553	187,395
Segment profit, Change in Contribution	$ 5,057		$ (4,842)
Segment profit, Change in Contribution (as a percent)	8.00%		(3.00%)
Segment assets:
Segment assets	$ 28,092,785	24,590,027	$ 28,092,785	24,590,027
Segment assets, Change in Contribution	$ 3,502,758
Segment assets, Change in Contribution (as a percent)	14.00%
Specialty finance
Segment profit:
Segment profit	$ 24,241	22,971	67,218	61,482
Segment profit, Change in Contribution	$ 1,270		$ 5,736
Segment profit, Change in Contribution (as a percent)	6.00%		9.00%
Segment assets:
Segment assets	$ 5,739,798	4,897,664	$ 5,739,798	4,897,664
Segment assets, Change in Contribution	$ 842,134
Segment assets, Change in Contribution (as a percent)	17.00%
Wealth management
Segment profit:
Segment profit	$ 6,088	5,242	19,962	14,632
Segment profit, Change in Contribution	$ 846		$ 5,330
Segment profit, Change in Contribution (as a percent)	16.00%		36.00%
Segment assets:
Segment assets	$ 1,079,319	655,040	$ 1,079,319	655,040
Segment assets, Change in Contribution	$ 424,279
Segment assets, Change in Contribution (as a percent)	65.00%
Operating Segments
Net interest income:
Net interest income	$ 259,340	242,881	776,614	696,759
Net interest income, Change in Contribution	$ 16,459		$ 79,855
Net interest income, Change in Contribution (as a percent)	7.00%		11.00%
Non-interest income:
Non-interest income	$ 127,295	110,274	$ 330,102	310,822
Non-interest income, Change in Contribution	$ 17,021		$ 19,280
Non-interest income, Change in Contribution (as a percent)	15.00%		6.00%
Net revenue:
Net revenue	$ 386,635	353,155	$ 1,106,716	1,007,581
Net revenue, Change in Contribution	$ 33,480		$ 99,135
Net revenue, Change in Contribution (as a percent)	9.00%		10.00%
Operating Segments | Community banking
Net interest income:
Net interest income	$ 209,912	202,435	$ 635,645	583,926
Net interest income, Change in Contribution	$ 7,477		$ 51,719
Net interest income, Change in Contribution (as a percent)	4.00%		9.00%
Non-interest income:
Non-interest income	$ 82,870	69,776	$ 196,507	192,028
Non-interest income, Change in Contribution	$ 13,094		$ 4,479
Non-interest income, Change in Contribution (as a percent)	19.00%		2.00%
Net revenue:
Net revenue	$ 292,782	272,211	$ 832,152	775,954
Net revenue, Change in Contribution	$ 20,571		$ 56,198
Net revenue, Change in Contribution (as a percent)	8.00%		7.00%
Operating Segments | Specialty finance
Net interest income:
Net interest income	$ 42,138	36,398	$ 119,241	100,104
Net interest income, Change in Contribution	$ 5,740		$ 19,137
Net interest income, Change in Contribution (as a percent)	16.00%		19.00%
Non-interest income:
Non-interest income	$ 20,003	16,963	$ 59,188	49,005
Non-interest income, Change in Contribution	$ 3,040		$ 10,183
Non-interest income, Change in Contribution (as a percent)	18.00%		21.00%
Net revenue:
Net revenue	$ 62,141	53,361	$ 178,429	149,109
Net revenue, Change in Contribution	$ 8,780		$ 29,320
Net revenue, Change in Contribution (as a percent)	16.00%		20.00%
Operating Segments | Wealth management
Net interest income:
Net interest income	$ 7,290	4,048	$ 21,728	12,729
Net interest income, Change in Contribution	$ 3,242		$ 8,999
Net interest income, Change in Contribution (as a percent)	80.00%		71.00%
Non-interest income:
Non-interest income	$ 24,422	23,535	$ 74,407	69,789
Non-interest income, Change in Contribution	$ 887		$ 4,618
Non-interest income, Change in Contribution (as a percent)	4.00%		7.00%
Net revenue:
Net revenue	$ 31,712	27,583	$ 96,135	82,518
Net revenue, Change in Contribution	$ 4,129		$ 13,617
Net revenue, Change in Contribution (as a percent)	15.00%		17.00%
Intersegment Eliminations
Net interest income:
Net interest income	$ 5,512	4,682	$ 16,426	14,056
Net interest income, Change in Contribution	$ 830		$ 2,370
Net interest income, Change in Contribution (as a percent)	18.00%		17.00%
Non-interest income:
Non-interest income	$ (12,158)	(10,344)	$ (35,150)	(29,980)
Non-interest income, Change in Contribution	$ (1,814)		$ (5,170)
Non-interest income, Change in Contribution (as a percent)	(18.00%)		(17.00%)
Net revenue:
Net revenue	$ (6,646)	$ (5,662)	$ (18,724)	$ (15,924)
Net revenue, Change in Contribution	$ (984)		$ (2,800)
Net revenue, Change in Contribution (as a percent)	(17.00%)		(18.00%)