Business Segments - Schedule of Financial Data for Business Segments (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Average interest earning assets (unaudited)	$ 27,702,911				$ 21,488,498				$ 27,702,911	$ 21,488,498	$ 19,829,312
Interest income	314,594	$ 297,041	$ 280,118	$ 267,495	217,951	$ 210,741	$ 207,007	$ 198,455	1,159,248	834,154	761,888
Interest expense	92,541	80,241	69,366	59,897	48,537	46,796	42,187	36,587	302,045	174,107	148,774
Net Interest Income	222,053	216,800	210,752	207,598	169,414	163,945	164,820	161,868	857,203	660,047	613,114
Provision for credit losses	7,859	6,552	7,142	10,948	2,200	1,640	3,632	2,470	32,501	9,942	11,869
Net Interest Income After Provision for Credit Losses									824,702	650,105	601,245
Non-interest income									134,052	111,706	108,260
Non-interest expense:									629,061	509,073	476,125
Internal expense transfer									0	0	0
Income Before Income Taxes	$ 95,176	$ 87,605	$ 91,763	$ 55,149	$ 61,056	$ 56,737	$ 70,779	$ 64,166	$ 329,693	$ 252,738	$ 233,380
Return on average interest earning assets (pre-tax)	1.19%				1.18%				1.19%	1.18%	1.18%
Corporate and Other Adjustments
Segment Reporting Information [Line Items]
Average interest earning assets (unaudited)	$ 0				$ 0				$ 0	$ 0	$ 0
Interest income									(5,961)	(8,623)	(8,760)
Interest expense									15,577	11,813	11,520
Net Interest Income									(21,538)	(20,436)	(20,280)
Provision for credit losses									0	0	0
Net Interest Income After Provision for Credit Losses									(21,538)	(20,436)	(20,280)
Non-interest income									41,806	29,172	29,796
Non-interest expense:									440,177	364,457	341,978
Internal expense transfer									(344,916)	(283,247)	(280,251)
Income Before Income Taxes									(74,993)	(72,474)	(52,211)
Consumer Lending | Operating Segments
Segment Reporting Information [Line Items]
Average interest earning assets (unaudited)	$ 6,197,161				$ 5,166,171				6,197,161	5,166,171	5,081,798
Interest income									235,264	182,508	176,929
Interest expense									64,083	39,018	35,175
Net Interest Income									171,181	143,490	141,754
Provision for credit losses									5,550	3,197	905
Net Interest Income After Provision for Credit Losses									165,631	140,293	140,849
Non-interest income									61,280	63,375	63,443
Non-interest expense:									92,462	72,207	62,721
Internal expense transfer									77,164	68,007	71,578
Income Before Income Taxes									$ 57,285	$ 63,454	$ 69,993
Return on average interest earning assets (pre-tax)	0.92%				1.23%				0.92%	1.23%	1.38%
Commercial Lending | Operating Segments
Segment Reporting Information [Line Items]
Average interest earning assets (unaudited)	$ 17,143,169				$ 12,652,832				$ 17,143,169	$ 12,652,832	$ 11,318,947
Interest income									798,974	552,297	504,341
Interest expense									177,273	95,562	78,347
Net Interest Income									621,701	456,735	425,994
Provision for credit losses									26,951	6,745	10,964
Net Interest Income After Provision for Credit Losses									594,750	449,990	415,030
Non-interest income									22,275	11,414	8,327
Non-interest expense:									95,171	71,216	70,145
Internal expense transfer									213,399	166,847	160,198
Income Before Income Taxes									$ 308,455	$ 223,341	$ 193,014
Return on average interest earning assets (pre-tax)	1.80%				1.77%				1.80%	1.77%	1.71%
Investment Management | Operating Segments
Segment Reporting Information [Line Items]
Average interest earning assets (unaudited)	$ 4,362,581				$ 3,669,495				$ 4,362,581	$ 3,669,495	$ 3,428,567
Interest income									130,971	107,972	89,378
Interest expense									45,112	27,714	23,732
Net Interest Income									85,859	80,258	65,646
Provision for credit losses									0	0	0
Net Interest Income After Provision for Credit Losses									85,859	80,258	65,646
Non-interest income									8,691	7,745	6,694
Non-interest expense:									1,251	1,193	1,281
Internal expense transfer									54,353	48,393	48,475
Income Before Income Taxes									$ 38,946	$ 38,417	$ 22,584
Return on average interest earning assets (pre-tax)	0.89%				1.05%				0.89%	1.05%	0.66%