Segment Information - Information about Company's Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Net Income (Loss)	$ 480,081	$ 526,091	$ 1,436,083	$ 1,371,861
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	1,557,669	1,488,332	4,662,070	4,388,746
Business Banking [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	46,147	43,730	131,668	124,455
Business Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	143,741	138,898	423,830	402,811
Business Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	912	920	2,772	2,750
Commercial Banking [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	132,668	147,910	388,393	399,766
Commercial Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	282,757	280,671	828,357	816,566
Commercial Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	980	875	2,707	2,653
Commercial Real Estate [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	131,161	116,386	370,288	337,380
Commercial Real Estate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	238,364	214,266	682,979	622,414
Commercial Real Estate [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	458	371	1,218	1,096
Discretionary Portfolio [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	29,604	39,723	106,816	88,575
Discretionary Portfolio [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	50,871	53,061	175,764	162,075
Discretionary Portfolio [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	(10,550)	(10,270)	(29,140)	(31,656)
Residential Mortgage Banking [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	15,846	13,289	36,429	43,637
Residential Mortgage Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	115,239	79,603	295,449	244,228
Residential Mortgage Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	19,363	15,402	51,027	46,870
Retail Banking [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	131,638	141,612	417,198	408,081
Retail Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	432,339	427,452	1,299,823	1,241,468
Retail Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	1,808	2,862	7,106	8,434
All Other [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	(6,983)	23,441	(14,709)	(30,033)
All Other [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	294,358	294,381	955,868	899,184
All Other [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	$ (12,971)	$ (10,160)	$ (35,690)	$ (30,147)