Segment Information - Information about Company's Segments (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Net interest income	$ 3,781,044	$ 3,469,887	$ 2,842,587
Noninterest income	1,851,143	1,825,996	1,825,037
Total income	5,632,187	5,295,883	4,667,624
Provision for credit losses	168,000	190,000	170,000
Amortization of core deposit and other intangible assets	31,366	42,613	26,424
Depreciation and other amortization	165,759	157,978	148,925
Other noninterest expense	2,943,200	2,846,894	2,647,583
Income before taxes	2,323,862	2,058,398	1,674,692
Income tax expense (benefit)	915,556	743,284	595,025
Net income	1,408,306	1,315,114	1,079,667
Average total assets	120,860,000	124,340,000	101,780,000
Capital expenditures	79,000	108,000	82,000
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income		3,469,887	2,842,587
All Other [Member]
Segment Reporting Information [Line Items]
Net interest income	410,928	221,151	64,590
Noninterest income	609,945	570,475	594,586
Total income	1,020,873	791,626	659,176
Provision for credit losses	8,265	(3,910)	62,074
Amortization of core deposit and other intangible assets	30,306	42,613	26,424
Depreciation and other amortization	69,923	68,541	64,852
Other noninterest expense	1,019,465	892,625	959,345
Income before taxes	(107,086)	(208,243)	(453,519)
Income tax expense (benefit)	(40,752)	(144,498)	(230,184)
Net income	(66,334)	(63,745)	(223,335)
Average total assets	13,684,000	16,885,000	12,870,000
Capital expenditures	44,000	62,000	68,000
Business Banking [Member]
Segment Reporting Information [Line Items]
Net income		103,758	109,744
Business Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	393,948	371,889	357,378
Noninterest income	112,512	108,783	108,195
Total income	506,460	480,672	465,573
Provision for credit losses	15,598	12,709	15,513
Depreciation and other amortization	393	404	407
Other noninterest expense	294,493	292,124	264,163
Income before taxes	195,976	175,435	185,490
Income tax expense (benefit)	80,043	71,677	75,746
Net income	115,933	103,758	109,744
Average total assets	5,602,000	5,456,000	5,339,000
Commercial Banking [Member]
Segment Reporting Information [Line Items]
Net income		411,379	430,886
Commercial Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	809,301	785,339	753,362
Noninterest income	283,447	274,923	290,142
Total income	1,092,748	1,060,262	1,043,504
Provision for credit losses	11,876	34,903	25,089
Depreciation and other amortization	509	520	566
Other noninterest expense	339,936	327,616	288,303
Income before taxes	740,427	697,223	729,546
Income tax expense (benefit)	303,556	285,844	298,660
Net income	436,871	411,379	430,886
Average total assets	26,573,000	25,592,000	24,143,000
Commercial Real Estate [Member]
Segment Reporting Information [Line Items]
Net income		350,358	340,641
Commercial Real Estate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	649,378	608,385	577,922
Noninterest income	169,966	179,706	142,948
Total income	819,344	788,091	720,870
Provision for credit losses	(7,524)	(3,447)	(8,003)
Amortization of core deposit and other intangible assets	1,060
Depreciation and other amortization	24,410	20,120	19,247
Other noninterest expense	207,493	204,965	169,688
Income before taxes	593,905	566,453	539,938
Income tax expense (benefit)	229,770	216,095	199,297
Net income	364,135	350,358	340,641
Average total assets	22,741,000	21,131,000	18,827,000
Capital expenditures	1,000
Discretionary Portfolio [Member]
Segment Reporting Information [Line Items]
Net income		163,538	59,272
Discretionary Portfolio [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	277,095	345,926	97,626
Noninterest income	23,851	26,075	28,114
Total income	300,946	372,001	125,740
Provision for credit losses	31,119	32,925	7,599
Depreciation and other amortization	279	472	679
Other noninterest expense	76,021	95,300	49,839
Income before taxes	193,527	243,304	67,623
Income tax expense (benefit)	58,559	79,766	8,351
Net income	134,968	163,538	59,272
Average total assets	37,203,000	40,867,000	26,648,000
Residential Mortgage Banking [Member]
Segment Reporting Information [Line Items]
Net income		55,453	74,362
Residential Mortgage Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	30,328	29,809	39,969
Noninterest income	321,589	342,858	336,099
Total income	351,917	372,667	376,068
Provision for credit losses	1,254	(3,617)	(5,225)
Depreciation and other amortization	32,011	30,264	27,883
Other noninterest expense	247,639	258,141	233,651
Income before taxes	71,013	87,879	119,759
Income tax expense (benefit)	25,446	32,426	45,397
Net income	45,567	55,453	74,362
Average total assets	2,355,000	2,569,000	2,918,000
Retail Banking [Member]
Segment Reporting Information [Line Items]
Net income		294,373	288,097
Retail Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net interest income	1,210,066	1,107,388	951,740
Noninterest income	329,833	323,176	324,953
Total income	1,539,899	1,430,564	1,276,693
Provision for credit losses	107,412	120,437	72,953
Depreciation and other amortization	38,234	37,657	35,291
Other noninterest expense	758,153	776,123	682,594
Income before taxes	636,100	496,347	485,855
Income tax expense (benefit)	258,934	201,974	197,758
Net income	377,166	294,373	288,097
Average total assets	12,702,000	11,840,000	11,035,000
Capital expenditures	$ 34,000	$ 46,000	$ 14,000