Segment Information - Information about Company's Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Net Income (Loss)	$ 526,091	$ 355,923	$ 1,371,861	$ 1,085,903
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	1,488,332	1,416,563	4,388,746	4,176,684
Business Banking [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	43,730	31,408	124,455	85,146
Business Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	138,898	130,501	402,811	375,816
Business Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	920	919	2,750	2,836
Commercial Banking [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	147,910	109,797	399,766	328,211
Commercial Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	280,671	267,822	816,566	818,846
Commercial Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	875	840	2,653	2,603
Commercial Real Estate [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	116,386	97,295	337,380	269,113
Commercial Real Estate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	214,266	212,014	622,414	604,437
Commercial Real Estate [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	371	401	1,096	1,165
Discretionary Portfolio [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	39,723	31,515	88,575	95,200
Discretionary Portfolio [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	53,061	65,808	162,075	216,798
Discretionary Portfolio [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	(10,270)	(12,346)	(31,656)	(37,670)
Residential Mortgage Banking [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	13,289	13,546	43,637	37,206
Residential Mortgage Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	79,603	90,018	244,228	264,120
Residential Mortgage Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	15,402	18,866	46,870	55,221
Retail Banking [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	141,612	96,329	408,081	282,614
Retail Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	427,452	392,542	1,241,468	1,144,964
Retail Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	2,862	2,668	8,434	8,760
All Other [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	23,441	(23,967)	(30,033)	(11,587)
All Other [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	294,381	257,858	899,184	751,703
All Other [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	$ (10,160)	$ (11,348)	$ (30,147)	$ (32,915)