Segment Information - Information about Company's Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Net Income (Loss)	$ 381,053	$ 336,031	$ 729,980	$ 634,559
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	1,399,016	1,312,073	2,760,121	2,604,970
Business Banking [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	28,750	25,254	53,738	53,316
Business Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	124,982	118,587	245,315	236,683
Business Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	1,006	1,197	1,917	2,188
Commercial Banking [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	105,627	105,311	218,414	206,560
Commercial Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	277,116	265,344	551,024	518,830
Commercial Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	843	911	1,763	1,967
Commercial Real Estate [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	87,271	84,088	171,818	164,617
Commercial Real Estate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	197,298	192,175	392,423	369,555
Commercial Real Estate [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	357	449	764	836
Discretionary Portfolio [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	29,740	40,613	63,685	89,024
Discretionary Portfolio [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	72,044	88,998	150,990	190,035
Discretionary Portfolio [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	(12,397)	(14,608)	(25,324)	(28,931)
Residential Mortgage Banking [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	13,742	14,329	23,660	27,190
Residential Mortgage Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	88,700	94,353	174,102	184,179
Residential Mortgage Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	18,144	21,244	36,355	40,904
Retail Banking [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	100,094	76,441	186,285	144,481
Retail Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	383,904	354,001	752,422	701,059
Retail Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	3,045	3,132	6,092	6,146
All Other [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	15,829	(10,005)	12,380	(50,629)
All Other [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	254,972	198,615	493,845	404,629
All Other [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	$ (10,998)	$ (12,325)	$ (21,567)	$ (23,110)