Segment Information (Summary of Segment Information) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Net interest income:
Net interest income	$ 238,170	$ 204,409	$ 463,252	$ 396,989
Net interest income, Change in Contribution	$ 33,761		$ 66,263
Net interest income, Change in Contribution (as a percent)	17.00%		17.00%
Non-interest income:
Non-interest income	$ 95,233	89,972	$ 180,912	158,737
Non-interest income, Change in Contribution	$ 5,261		$ 22,175
Non-interest income, Change in Contribution (as a percent)	6.00%		14.00%
Net revenue:
Net revenue	$ 333,403	294,381	$ 644,164	555,726
Net revenue, Change in Contribution	$ 39,022		$ 88,438
Net revenue, Change in Contribution (as a percent)	13.00%		16.00%
Segment profit:
Segment profit	$ 89,580	64,897	$ 171,561	123,275
Segment profit, Change in Contribution	$ 24,683		$ 48,286
Segment profit, Change in Contribution (as a percent)	38.00%		39.00%
Segment assets:
Segment assets	$ 29,464,588	26,929,265	$ 29,464,588	26,929,265	$ 27,915,970
Segment assets, Change in Contribution	$ 2,535,323
Segment assets, Change in Contribution (as a percent)	9.00%
Community banking
Segment profit:
Segment profit	$ 65,355	46,026	123,660	83,703
Segment profit, Change in Contribution	$ 19,329		$ 39,957
Segment profit, Change in Contribution (as a percent)	42.00%		48.00%
Segment assets:
Segment assets	$ 23,979,910	22,032,302	$ 23,979,910	22,032,302
Segment assets, Change in Contribution	$ 1,947,608
Segment assets, Change in Contribution (as a percent)	9.00%
Specialty finance
Segment profit:
Segment profit	$ 19,536	14,849	38,511	30,947
Segment profit, Change in Contribution	$ 4,687		$ 7,564
Segment profit, Change in Contribution (as a percent)	32.00%		24.00%
Segment assets:
Segment assets	$ 4,824,307	4,255,109	$ 4,824,307	4,255,109
Segment assets, Change in Contribution	$ 569,198
Segment assets, Change in Contribution (as a percent)	13.00%
Wealth management
Segment profit:
Segment profit	$ 4,689	4,022	9,390	8,625
Segment profit, Change in Contribution	$ 667		$ 765
Segment profit, Change in Contribution (as a percent)	17.00%		9.00%
Segment assets:
Segment assets	$ 660,371	641,854	$ 660,371	641,854
Segment assets, Change in Contribution	$ 18,517
Segment assets, Change in Contribution (as a percent)	3.00%
Operating Segments
Net interest income:
Net interest income	$ 233,271	199,806	453,878	387,954
Net interest income, Change in Contribution	$ 33,465		$ 65,924
Net interest income, Change in Contribution (as a percent)	17.00%		17.00%
Non-interest income:
Non-interest income	$ 105,318	99,301	$ 200,548	176,975
Non-interest income, Change in Contribution	$ 6,017		$ 23,573
Non-interest income, Change in Contribution (as a percent)	6.00%		13.00%
Net revenue:
Net revenue	$ 338,589	299,107	$ 654,426	564,929
Net revenue, Change in Contribution	$ 39,482		$ 89,497
Net revenue, Change in Contribution (as a percent)	13.00%		16.00%
Operating Segments | Community banking
Net interest income:
Net interest income	$ 196,849	166,329	$ 381,491	322,609
Net interest income, Change in Contribution	$ 30,520		$ 58,882
Net interest income, Change in Contribution (as a percent)	18.00%		18.00%
Non-interest income:
Non-interest income	$ 65,705	65,007	$ 122,252	107,723
Non-interest income, Change in Contribution	$ 698		$ 14,529
Non-interest income, Change in Contribution (as a percent)	1.00%		13.00%
Net revenue:
Net revenue	$ 262,554	231,336	$ 503,743	430,332
Net revenue, Change in Contribution	$ 31,218		$ 73,411
Net revenue, Change in Contribution (as a percent)	13.00%		17.00%
Operating Segments | Specialty finance
Net interest income:
Net interest income	$ 32,182	28,558	$ 63,706	55,370
Net interest income, Change in Contribution	$ 3,624		$ 8,336
Net interest income, Change in Contribution (as a percent)	13.00%		15.00%
Non-interest income:
Non-interest income	$ 16,317	13,721	$ 32,042	27,877
Non-interest income, Change in Contribution	$ 2,596		$ 4,165
Non-interest income, Change in Contribution (as a percent)	19.00%		15.00%
Net revenue:
Net revenue	$ 48,499	42,279	$ 95,748	83,247
Net revenue, Change in Contribution	$ 6,220		$ 12,501
Net revenue, Change in Contribution (as a percent)	15.00%		15.00%
Operating Segments | Wealth management
Net interest income:
Net interest income	$ 4,240	4,919	$ 8,681	9,975
Net interest income, Change in Contribution	$ (679)		$ (1,294)
Net interest income, Change in Contribution (as a percent)	(14.00%)		(13.00%)
Non-interest income:
Non-interest income	$ 23,296	20,573	$ 46,254	41,375
Non-interest income, Change in Contribution	$ 2,723		$ 4,879
Non-interest income, Change in Contribution (as a percent)	13.00%		12.00%
Net revenue:
Net revenue	$ 27,536	25,492	$ 54,935	51,350
Net revenue, Change in Contribution	$ 2,044		$ 3,585
Net revenue, Change in Contribution (as a percent)	8.00%		7.00%
Intersegment Eliminations
Net interest income:
Net interest income	$ 4,899	4,603	$ 9,374	9,035
Net interest income, Change in Contribution	$ 296		$ 339
Net interest income, Change in Contribution (as a percent)	6.00%		4.00%
Non-interest income:
Non-interest income	$ (10,085)	(9,329)	$ (19,636)	(18,238)
Non-interest income, Change in Contribution	$ (756)		$ (1,398)
Non-interest income, Change in Contribution (as a percent)	(8.00%)		(8.00%)
Net revenue:
Net revenue	$ (5,186)	$ (4,726)	$ (10,262)	$ (9,203)
Net revenue, Change in Contribution	$ (460)		$ (1,059)
Net revenue, Change in Contribution (as a percent)	(10.00%)		(12.00%)