Segment Information - Information about Company's Segments (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Net interest income	$ 3,866,317	$ 4,130,264	$ 4,072,302
Noninterest income	2,088,444	2,061,679	1,856,000
Total income	5,954,761	6,191,943	5,928,302
Provision for credit losses	800,000	176,000	132,000
Amortization of core deposit and other intangible assets	14,869	19,490	24,522
Depreciation and other amortization	305,419	281,825	154,483
Other noninterest expense	3,064,952	3,167,367	3,109,057
Income before taxes	1,769,521	2,547,261	2,508,240
Income tax expense (benefit)	416,369	618,112	590,160
Net income	1,353,152	1,929,149	1,918,080
Average total assets	135,480,000	119,584,000	116,959,000
Capital expenditures	172,000	178,000	98,000
All Other [Member]
Segment Reporting Information [Line Items]
Net interest income	121,808	537,940	557,542
Noninterest income	725,472	695,443	651,439
Total income	847,280	1,233,383	1,208,981
Provision for credit losses	482,202	6,257	(8,128)
Amortization of core deposit and other intangible assets	13,809	18,430	23,462
Depreciation and other amortization	116,979	108,604	68,004
Other noninterest expense	959,258	1,117,668	1,125,428
Income before taxes	(724,968)	(17,576)	215
Income tax expense (benefit)	(202,396)	(28,625)	(54,766)
Net income	(522,572)	11,049	54,981
Average total assets	22,996,000	14,953,000	13,877,000
Capital expenditures	138,000	98,000	65,000
Business Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	462,614	451,307	434,579
Noninterest income	103,837	113,855	111,600
Total income	566,451	565,162	546,179
Provision for credit losses	25,928	16,501	10,916
Depreciation and other amortization	1,482	2,066	382
Other noninterest expense	322,868	317,482	305,340
Income before taxes	216,173	229,113	229,541
Income tax expense (benefit)	56,953	60,617	61,279
Net income	159,220	168,496	168,262
Average total assets	8,152,000	5,793,000	5,631,000
Capital expenditures		1,000
Commercial Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	864,149	828,888	821,812
Noninterest income	276,791	289,558	288,908
Total income	1,140,940	1,118,446	1,110,720
Provision for credit losses	73,099	25,580	8,976
Depreciation and other amortization	2,421	2,353	496
Other noninterest expense	375,769	382,214	364,102
Income before taxes	689,651	708,299	737,146
Income tax expense (benefit)	181,179	187,835	198,229
Net income	508,472	520,464	538,917
Average total assets	30,338,000	28,142,000	26,626,000
Capital expenditures		2,000
Commercial Real Estate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	673,894	692,526	665,220
Noninterest income	208,367	214,970	183,955
Total income	882,261	907,496	849,175
Provision for credit losses	107,210	1,537	3,159
Amortization of core deposit and other intangible assets	1,060	1,060	1,060
Depreciation and other amortization	28,187	26,963	25,852
Other noninterest expense	256,428	239,333	217,387
Income before taxes	489,376	638,603	601,717
Income tax expense (benefit)	107,548	152,977	148,807
Net income	381,828	485,626	452,910
Average total assets	25,792,000	23,921,000	22,885,000
Discretionary Portfolio [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	486,831	209,807	228,051
Noninterest income	(1,735)	26,919	(9,690)
Total income	485,096	236,726	218,361
Provision for credit losses	1,508	3,608	6,683
Depreciation and other amortization	285	279	187
Other noninterest expense	54,339	52,885	65,393
Income before taxes	428,964	179,954	146,098
Income tax expense (benefit)	101,673	36,342	29,872
Net income	327,291	143,612	116,226
Average total assets	27,726,000	29,081,000	32,123,000
Capital expenditures			1,000
Residential Mortgage Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	52,712	20,008	13,933
Noninterest income	515,549	393,372	305,560
Total income	568,261	413,380	319,493
Provision for credit losses	1,785	382	(2,178)
Depreciation and other amortization	60,129	48,248	24,288
Other noninterest expense	332,028	273,067	241,624
Income before taxes	174,319	91,683	55,759
Income tax expense (benefit)	40,667	19,355	10,272
Net income	133,652	72,328	45,487
Average total assets	4,038,000	2,611,000	2,161,000
Capital expenditures		1,000	1,000
Retail Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	1,204,309	1,389,788	1,351,165
Noninterest income	260,163	327,562	324,228
Total income	1,464,472	1,717,350	1,675,393
Provision for credit losses	108,268	122,135	112,572
Depreciation and other amortization	95,936	93,312	35,274
Other noninterest expense	764,262	784,718	789,783
Income before taxes	496,006	717,185	737,764
Income tax expense (benefit)	130,745	189,611	196,467
Net income	365,261	527,574	541,297
Average total assets	16,438,000	15,083,000	13,656,000
Capital expenditures	$ 34,000	$ 76,000	$ 31,000