Segment Information - Information about Company's Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Net income	$ 495,460	$ 372,136	$ 1,400,778	$ 882,012
Net Income (Loss)	495,460	372,136	1,400,778	882,012
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	1,536,376	1,463,656	4,479,342	4,414,231
Business Banking [Member]
Segment Reporting Information [Line Items]
Net income	72,017	35,614	159,895	106,103
Net Income (Loss)	72,017	35,614	159,895	106,103
Business Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	181,873	131,566	482,874	407,485
Business Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	694	(1)	2,157	1,028
Commercial Banking [Member]
Segment Reporting Information [Line Items]
Net income	144,367	131,414	378,040	386,176
Net Income (Loss)	144,367	131,414	378,040	386,176
Commercial Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	295,209	276,125	865,780	855,426
Commercial Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	1,009	117	2,830	1,419
Commercial Real Estate [Member]
Segment Reporting Information [Line Items]
Net income	84,663	87,095	242,625	311,586
Net Income (Loss)	84,663	87,095	242,625	311,586
Commercial Real Estate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	228,317	202,987	629,719	653,405
Commercial Real Estate [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	227	(17)	679	452
Discretionary Portfolio [Member]
Segment Reporting Information [Line Items]
Net income	74,666	97,399	243,744	218,758
Net Income (Loss)	74,666	97,399	243,744	218,758
Discretionary Portfolio [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	112,529	140,135	365,380	328,181
Discretionary Portfolio [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	(14,448)	(10,748)	(34,554)	(33,646)
Residential Mortgage Banking [Member]
Segment Reporting Information [Line Items]
Net income	46,077	45,318	125,791	107,334
Net Income (Loss)	46,077	45,318	125,791	107,334
Residential Mortgage Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	159,213	161,151	459,655	428,757
Residential Mortgage Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	25,150	22,190	70,208	64,623
Retail Banking [Member]
Segment Reporting Information [Line Items]
Net income	88,004	85,229	262,562	281,973
Net Income (Loss)	88,004	85,229	262,562	281,973
Retail Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	357,335	351,312	1,055,240	1,107,516
Retail Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	(53)	268	500	802
All Other [Member]
Segment Reporting Information [Line Items]
Net income	(14,334)	(109,933)	(11,879)	(529,918)
Net Income (Loss)	(14,334)	(109,933)	(11,879)	(529,918)
All Other [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	201,900	200,380	620,694	633,461
All Other [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	$ (12,579)	$ (11,809)	$ (41,820)	$ (34,678)