Segment Information - Information about Company's Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Net income	$ 372,136	$ 480,081	$ 882,012	$ 1,436,083
Net Income (Loss)	372,136	480,081	882,012	1,436,083
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	1,463,656	1,557,669	4,414,231	4,662,070
Business Banking [Member]
Segment Reporting Information [Line Items]
Net income	35,614	46,147	106,103	131,668
Net Income (Loss)	35,614	46,147	106,103	131,668
Business Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	131,566	143,741	407,485	423,830
Business Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	(1)	912	1,028	2,772
Commercial Banking [Member]
Segment Reporting Information [Line Items]
Net income	131,414	132,668	386,176	388,393
Net Income (Loss)	131,414	132,668	386,176	388,393
Commercial Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	276,125	282,757	855,426	828,357
Commercial Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	117	980	1,419	2,707
Commercial Real Estate [Member]
Segment Reporting Information [Line Items]
Net income	87,095	131,161	311,586	370,288
Net Income (Loss)	87,095	131,161	311,586	370,288
Commercial Real Estate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	202,987	238,364	653,405	682,979
Commercial Real Estate [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	(17)	458	452	1,218
Discretionary Portfolio [Member]
Segment Reporting Information [Line Items]
Net income	97,399	29,604	218,758	106,816
Net Income (Loss)	97,399	29,604	218,758	106,816
Discretionary Portfolio [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	140,135	50,871	328,181	175,764
Discretionary Portfolio [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	(10,748)	(10,550)	(33,646)	(29,140)
Residential Mortgage Banking [Member]
Segment Reporting Information [Line Items]
Net income	45,318	15,846	107,334	36,429
Net Income (Loss)	45,318	15,846	107,334	36,429
Residential Mortgage Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	161,151	115,239	428,757	295,449
Residential Mortgage Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	22,190	19,363	64,623	51,027
Retail Banking [Member]
Segment Reporting Information [Line Items]
Net income	85,229	131,638	281,973	417,198
Net Income (Loss)	85,229	131,638	281,973	417,198
Retail Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	351,312	432,339	1,107,516	1,299,823
Retail Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	268	1,808	802	7,106
All Other [Member]
Segment Reporting Information [Line Items]
Net income	(109,933)	(6,983)	(529,918)	(14,709)
Net Income (Loss)	(109,933)	(6,983)	(529,918)	(14,709)
All Other [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	200,380	294,358	633,461	955,868
All Other [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	$ (11,809)	$ (12,971)	$ (34,678)	$ (35,690)