Segment Information - Information about Company's Segments (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Net interest income	$ 4,072,302	$ 3,781,044	$ 3,469,887
Noninterest income	1,856,000	1,851,143	1,825,996
Total income	5,928,302	5,632,187	5,295,883
Provision for credit losses	132,000	168,000	190,000
Amortization of core deposit and other intangible assets	24,522	31,366	42,613
Depreciation and other amortization	154,483	165,759	157,978
Other noninterest expense	3,109,057	2,943,200	2,846,894
Income before taxes	2,508,240	2,323,862	2,058,398
Income tax expense (benefit)	590,160	915,556	743,284
Net income	1,918,080	1,408,306	1,315,114
Average total assets	116,959,000	120,860,000	124,340,000
Capital expenditures	98,000	79,000	108,000
All Other [Member]
Segment Reporting Information [Line Items]
Net interest income	557,542	410,928	221,151
Noninterest income	651,439	609,945	570,475
Total income	1,208,981	1,020,873	791,626
Provision for credit losses	(8,128)	8,265	(3,910)
Amortization of core deposit and other intangible assets	23,462	30,306	42,613
Depreciation and other amortization	68,004	69,923	68,541
Other noninterest expense	1,125,428	1,019,465	892,625
Income before taxes	215	(107,086)	(208,243)
Income tax expense (benefit)	(54,766)	(40,752)	(144,498)
Net income	54,981	(66,334)	(63,745)
Average total assets	13,877,000	13,684,000	16,885,000
Capital expenditures	65,000	44,000	62,000
Business Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	434,579	393,948	371,889
Noninterest income	111,600	112,512	108,783
Total income	546,179	506,460	480,672
Provision for credit losses	10,916	15,598	12,709
Depreciation and other amortization	382	393	404
Other noninterest expense	305,340	294,493	292,124
Income before taxes	229,541	195,976	175,435
Income tax expense (benefit)	61,279	80,043	71,677
Net income	168,262	115,933	103,758
Average total assets	5,631,000	5,602,000	5,456,000
Commercial Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	821,812	809,301	785,339
Noninterest income	288,908	283,447	274,923
Total income	1,110,720	1,092,748	1,060,262
Provision for credit losses	8,976	11,876	34,903
Depreciation and other amortization	496	509	520
Other noninterest expense	364,102	339,936	327,616
Income before taxes	737,146	740,427	697,223
Income tax expense (benefit)	198,229	303,556	285,844
Net income	538,917	436,871	411,379
Average total assets	26,626,000	26,573,000	25,592,000
Commercial Real Estate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	665,220	649,378	608,385
Noninterest income	183,955	169,966	179,706
Total income	849,175	819,344	788,091
Provision for credit losses	3,159	(7,524)	(3,447)
Amortization of core deposit and other intangible assets	1,060	1,060
Depreciation and other amortization	25,852	24,410	20,120
Other noninterest expense	217,387	207,493	204,965
Income before taxes	601,717	593,905	566,453
Income tax expense (benefit)	148,807	229,770	216,095
Net income	452,910	364,135	350,358
Average total assets	22,885,000	22,741,000	21,131,000
Capital expenditures		1,000
Discretionary Portfolio [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	228,051	277,095	345,926
Noninterest income	(9,690)	23,851	26,075
Total income	218,361	300,946	372,001
Provision for credit losses	6,683	31,119	32,925
Depreciation and other amortization	187	279	472
Other noninterest expense	65,393	76,021	95,300
Income before taxes	146,098	193,527	243,304
Income tax expense (benefit)	29,872	58,559	79,766
Net income	116,226	134,968	163,538
Average total assets	32,123,000	37,203,000	40,867,000
Capital expenditures	1,000
Residential Mortgage Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	13,933	30,328	29,809
Noninterest income	305,560	321,589	342,858
Total income	319,493	351,917	372,667
Provision for credit losses	(2,178)	1,254	(3,617)
Depreciation and other amortization	24,288	32,011	30,264
Other noninterest expense	241,624	247,639	258,141
Income before taxes	55,759	71,013	87,879
Income tax expense (benefit)	10,272	25,446	32,426
Net income	45,487	45,567	55,453
Average total assets	2,161,000	2,355,000	2,569,000
Capital expenditures	1,000
Retail Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	1,351,165	1,210,066	1,107,388
Noninterest income	324,228	329,833	323,176
Total income	1,675,393	1,539,899	1,430,564
Provision for credit losses	112,572	107,412	120,437
Depreciation and other amortization	35,274	38,234	37,657
Other noninterest expense	789,783	758,153	776,123
Income before taxes	737,764	636,100	496,347
Income tax expense (benefit)	196,467	258,934	201,974
Net income	541,297	377,166	294,373
Average total assets	13,656,000	12,702,000	11,840,000
Capital expenditures	$ 31,000	$ 34,000	$ 46,000