Segment Information - Information about Company's Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Net Income (Loss)	$ 493,160	$ 381,053	$ 845,770	$ 729,980
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	1,466,201	1,399,016	2,900,414	2,760,121
Business Banking [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	43,007	28,750	80,725	53,738
Business Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	136,029	124,982	263,913	245,315
Business Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	912	1,006	1,830	1,917
Commercial Banking [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	126,391	105,627	251,856	218,414
Commercial Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	269,499	277,116	535,895	551,024
Commercial Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	928	843	1,778	1,763
Commercial Real Estate [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	112,691	87,271	220,994	171,818
Commercial Real Estate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	205,541	197,298	408,148	392,423
Commercial Real Estate [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	385	357	725	764
Discretionary Portfolio [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	29,113	29,740	48,852	63,685
Discretionary Portfolio [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	58,976	72,044	109,014	150,990
Discretionary Portfolio [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	(10,554)	(12,397)	(21,386)	(25,324)
Residential Mortgage Banking [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	15,402	13,742	30,348	23,660
Residential Mortgage Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	82,167	88,700	164,625	174,102
Residential Mortgage Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	16,047	18,144	31,468	36,355
Retail Banking [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	143,089	100,094	266,469	186,285
Retail Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	420,540	383,904	814,016	752,422
Retail Banking [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	2,707	3,045	5,572	6,092
All Other [Member]
Segment Reporting Information [Line Items]
Net Income (Loss)	23,467	15,829	(53,474)	12,380
All Other [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues	293,449	254,972	604,803	493,845
All Other [Member] | Intersegment Activity Eliminated in Consolidated Totals [Member]
Segment Reporting Information [Line Items]
Total Revenues	$ (10,425)	$ (10,998)	$ (19,987)	$ (21,567)