Business Segments - Schedule of Financial Data for Business Segments (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Average interest earning assets (unaudited)	$ 30,575,530				$ 27,702,911				$ 30,575,530	$ 27,702,911	$ 21,488,498
Interest income	343,773	$ 329,261	$ 327,742	$ 320,224	314,594	$ 297,041	$ 280,118	$ 267,495	1,321,000	1,159,248	834,154
Interest expense	105,232	108,636	107,508	101,576	92,541	80,241	69,366	59,897	422,952	302,045	174,107
Net Interest Income	238,541	220,625	220,234	218,648	222,053	216,800	210,752	207,598	898,048	857,203	660,047
Provision for credit losses	5,418	8,700	2,100	8,000	7,859	6,552	7,142	10,948	24,218	32,501	9,942
Net Interest Income After Provision for Credit Losses									873,830	824,702	650,105
Non-interest income									214,520	134,052	111,706
Non-interest expense:									631,555	629,061	509,073
Internal expense transfer									0	0	0
Income Before Income Taxes	$ 75,071	$ 107,198	$ 104,000	$ 170,526	$ 95,176	$ 87,605	$ 91,763	$ 55,149	$ 456,795	$ 329,693	$ 252,738
Return on average interest earning assets (pre-tax)	1.49%				1.19%				1.49%	1.19%	1.18%
Corporate and Other Adjustments
Segment Reporting Information [Line Items]
Average interest earning assets (unaudited)	$ 0				$ 0				$ 0	$ 0	$ 0
Interest income									(4,824)	(5,961)	(8,623)
Interest expense									15,669	15,577	11,813
Net Interest Income									(20,493)	(21,538)	(20,436)
Provision for credit losses									0	0	0
Net Interest Income After Provision for Credit Losses									(20,493)	(21,538)	(20,436)
Non-interest income									106,564	41,806	29,172
Non-interest expense:									452,551	440,177	364,457
Internal expense transfer									(349,526)	(344,916)	(283,247)
Income Before Income Taxes									(16,954)	(74,993)	(72,474)
Consumer Lending | Operating Segments
Segment Reporting Information [Line Items]
Average interest earning assets (unaudited)	$ 6,891,462				$ 6,197,161				6,891,462	6,197,161	5,166,171
Interest income									272,773	235,264	182,508
Interest expense									91,798	64,083	39,018
Net Interest Income									180,975	171,181	143,490
Provision for credit losses									6,688	5,550	3,197
Net Interest Income After Provision for Credit Losses									174,287	165,631	140,293
Non-interest income									57,981	61,280	63,375
Non-interest expense:									76,046	92,462	72,207
Internal expense transfer									78,743	77,164	68,007
Income Before Income Taxes									$ 77,479	$ 57,285	$ 63,454
Return on average interest earning assets (pre-tax)	1.12%				0.92%				1.12%	0.92%	1.23%
Commercial Lending | Operating Segments
Segment Reporting Information [Line Items]
Average interest earning assets (unaudited)	$ 19,343,791				$ 17,143,169				$ 19,343,791	$ 17,143,169	$ 12,652,832
Interest income									926,328	798,974	552,297
Interest expense									257,670	177,273	95,562
Net Interest Income									668,658	621,701	456,735
Provision for credit losses									17,530	26,951	6,745
Net Interest Income After Provision for Credit Losses									651,128	594,750	449,990
Non-interest income									41,157	22,275	11,414
Non-interest expense:									101,924	95,171	71,216
Internal expense transfer									221,113	213,399	166,847
Income Before Income Taxes									$ 369,248	$ 308,455	$ 223,341
Return on average interest earning assets (pre-tax)	1.91%				1.80%				1.91%	1.80%	1.77%
Investment Management | Operating Segments
Segment Reporting Information [Line Items]
Average interest earning assets (unaudited)	$ 4,340,277				$ 4,362,581				$ 4,340,277	$ 4,362,581	$ 3,669,495
Interest income									126,723	130,971	107,972
Interest expense									57,815	45,112	27,714
Net Interest Income									68,908	85,859	80,258
Provision for credit losses									0	0	0
Net Interest Income After Provision for Credit Losses									68,908	85,859	80,258
Non-interest income									8,818	8,691	7,745
Non-interest expense:									1,034	1,251	1,193
Internal expense transfer									49,670	54,353	48,393
Income Before Income Taxes									$ 27,022	$ 38,946	$ 38,417
Return on average interest earning assets (pre-tax)	0.62%				0.89%				0.62%	0.89%	1.05%