Segment Information - Information about Company's Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Net income	$ 458,069	$ 241,054	$ 905,318	$ 509,876
Net Income (Loss)	458,069	241,054	905,318	509,876
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	1,455,973	1,444,410	2,942,966	2,950,575
Business Banking [Member]
Segment Reporting Information [Line Items]
Net income	41,607	37,563	87,878	70,489
Net Income (Loss)	41,607	37,563	87,878	70,489
Business Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	145,098	138,086	301,001	275,919
Business Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	794	144	1,463	1,029
Commercial Banking [Member]
Segment Reporting Information [Line Items]
Net income	111,035	111,030	233,673	254,762
Net Income (Loss)	111,035	111,030	233,673	254,762
Commercial Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	284,659	287,525	570,571	579,301
Commercial Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	1,012	249	1,821	1,302
Commercial Real Estate [Member]
Segment Reporting Information [Line Items]
Net income	86,690	107,083	157,962	224,491
Net Income (Loss)	86,690	107,083	157,962	224,491
Commercial Real Estate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	200,559	227,113	401,402	450,418
Commercial Real Estate [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	230	60	452	469
Discretionary Portfolio [Member]
Segment Reporting Information [Line Items]
Net income	78,532	95,691	169,078	121,359
Net Income (Loss)	78,532	95,691	169,078	121,359
Discretionary Portfolio [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	118,307	138,827	252,851	188,046
Discretionary Portfolio [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	(10,079)	(9,861)	(20,106)	(22,898)
Residential Mortgage Banking [Member]
Segment Reporting Information [Line Items]
Net income	30,103	37,400	79,714	62,016
Net Income (Loss)	30,103	37,400	79,714	62,016
Residential Mortgage Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	146,682	142,045	300,442	267,606
Residential Mortgage Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	22,875	21,016	45,058	42,433
Retail Banking [Member]
Segment Reporting Information [Line Items]
Net income	89,200	86,498	174,558	196,744
Net Income (Loss)	89,200	86,498	174,558	196,744
Retail Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	350,988	353,521	697,905	756,204
Retail Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	281	267	553	534
All Other [Member]
Segment Reporting Information [Line Items]
Net income	20,902	(234,211)	2,455	(419,985)
Net Income (Loss)	20,902	(234,211)	2,455	(419,985)
All Other [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	209,680	157,293	418,794	433,081
All Other [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	$ (15,113)	$ (11,875)	$ (29,241)	$ (22,869)