Business Segments - Schedule of Financial Data for Business Segments (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Average interest earning assets (unaudited)	$ 21,488,498				$ 19,829,312				$ 21,488,498	$ 19,829,312	$ 17,425,504
Interest income	220,506	$ 211,650	$ 211,147	$ 199,116	201,095	$ 191,203	$ 189,028	$ 185,597	842,419	766,923	707,023
Interest expense	48,537	46,796	42,187	36,587	36,700	37,057	37,573	37,444	174,107	148,774	156,754
Net Interest Income	171,969	164,854	168,960	162,529	164,395	154,146	151,455	148,153	668,312	618,149	550,269
Provision for credit losses	2,200	1,640	3,632	2,470	3,800	5,840	1,429	800	9,942	11,869	8,101
Net Interest Income After Provision for Credit Losses									658,370	606,280	542,168
Non-interest income									103,441	103,225	83,802
Non-interest expense:									509,073	476,125	499,075
Internal expense transfer									0	0	0
Income Before Income Taxes	$ 61,056	$ 56,737	$ 70,779	$ 64,166	$ 68,426	$ 59,891	$ 54,487	$ 50,576	$ 252,738	$ 233,380	$ 126,895
Return on average interest earning assets (pre-tax)	1.18%				1.18%				1.18%	1.18%	0.73%
Corporate and Other Adjustments
Segment Reporting Information [Line Items]
Average interest earning assets (unaudited)	$ 0				$ 0				$ 0	$ 0	$ 0
Interest income									(8,623)	(8,760)	(8,277)
Interest expense									11,813	11,520	11,233
Net Interest Income									(20,436)	(20,280)	(19,510)
Provision for credit losses									0	0	0
Net Interest Income After Provision for Credit Losses									(20,436)	(20,280)	(19,510)
Non-interest income									29,172	29,796	30,937
Non-interest expense:									364,457	341,978	370,051
Internal expense transfer									(283,247)	(280,251)	(264,364)
Income Before Income Taxes									(72,474)	(52,211)	(94,260)
Consumer Lending | Operating Segments
Segment Reporting Information [Line Items]
Average interest earning assets (unaudited)	$ 5,166,171				$ 5,081,798				5,166,171	5,081,798	4,764,306
Interest income									182,508	176,929	170,569
Interest expense									39,018	35,175	39,787
Net Interest Income									143,490	141,754	130,782
Provision for credit losses									3,197	905	1,153
Net Interest Income After Provision for Credit Losses									140,293	140,849	129,629
Non-interest income									63,375	63,443	45,306
Non-interest expense:									72,207	62,721	59,794
Internal expense transfer									68,007	71,578	72,441
Income Before Income Taxes									$ 63,454	$ 69,993	$ 42,700
Return on average interest earning assets (pre-tax)	1.23%				1.38%				1.23%	1.38%	0.90%
Commercial Lending | Operating Segments
Segment Reporting Information [Line Items]
Average interest earning assets (unaudited)	$ 12,652,832				$ 11,318,947				$ 12,652,832	$ 11,318,947	$ 9,682,714
Interest income									560,562	509,376	463,062
Interest expense									95,562	78,347	80,861
Net Interest Income									465,000	431,029	382,201
Provision for credit losses									6,745	10,964	6,948
Net Interest Income After Provision for Credit Losses									458,255	420,065	375,253
Non-interest income									3,149	3,292	744
Non-interest expense:									71,216	70,145	68,156
Internal expense transfer									166,847	160,198	146,463
Income Before Income Taxes									$ 223,341	$ 193,014	$ 161,378
Return on average interest earning assets (pre-tax)	1.77%				1.71%				1.77%	1.71%	1.67%
Investment Management | Operating Segments
Segment Reporting Information [Line Items]
Average interest earning assets (unaudited)	$ 3,669,495				$ 3,428,567				$ 3,669,495	$ 3,428,567	$ 2,978,484
Interest income									107,972	89,378	81,669
Interest expense									27,714	23,732	24,873
Net Interest Income									80,258	65,646	56,796
Provision for credit losses									0	0	0
Net Interest Income After Provision for Credit Losses									80,258	65,646	56,796
Non-interest income									7,745	6,694	6,815
Non-interest expense:									1,193	1,281	1,074
Internal expense transfer									48,393	48,475	45,460
Income Before Income Taxes									$ 38,417	$ 22,584	$ 17,077
Return on average interest earning assets (pre-tax)	1.05%				0.66%				1.05%	0.66%	0.57%