Segment Information - Summary of Reclassified Segment Information on Comparable Basis (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Net Interest Income	$ 3,781,044	$ 3,469,887	$ 2,842,587
Net income	1,408,306	1,315,114	1,079,667
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Interest Income		3,469,887	2,842,587
Business Banking [Member]
Segment Reporting Information [Line Items]
Net income		103,758	109,744
Business Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Interest Income	393,948	371,889	357,378
Net income	115,933	103,758	109,744
Commercial Banking [Member]
Segment Reporting Information [Line Items]
Net income		411,379	430,886
Commercial Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Interest Income	809,301	785,339	753,362
Net income	436,871	411,379	430,886
Commercial Real Estate [Member]
Segment Reporting Information [Line Items]
Net income		350,358	340,641
Commercial Real Estate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Interest Income	649,378	608,385	577,922
Net income	364,135	350,358	340,641
Discretionary Portfolio [Member]
Segment Reporting Information [Line Items]
Net income		163,538	59,272
Discretionary Portfolio [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Interest Income	277,095	345,926	97,626
Net income	134,968	163,538	59,272
Residential Mortgage Banking [Member]
Segment Reporting Information [Line Items]
Net income		55,453	74,362
Residential Mortgage Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Interest Income	30,328	29,809	39,969
Net income	45,567	55,453	74,362
Retail Banking [Member]
Segment Reporting Information [Line Items]
Net income		294,373	288,097
Retail Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Interest Income	1,210,066	1,107,388	951,740
Net income	$ 377,166	294,373	288,097
All Other [Member]
Segment Reporting Information [Line Items]
Net income		(63,745)	(223,335)
All Other [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Interest Income		221,151	64,590
As Previously Reported [Member]
Segment Reporting Information [Line Items]
Net income		1,315,114	1,079,667
As Previously Reported [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Interest Income		3,469,887	2,842,587
As Previously Reported [Member] | Business Banking [Member]
Segment Reporting Information [Line Items]
Net income		93,346	98,758
As Previously Reported [Member] | Business Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Interest Income		354,333	338,855
As Previously Reported [Member] | Commercial Banking [Member]
Segment Reporting Information [Line Items]
Net income		411,696	431,030
As Previously Reported [Member] | Commercial Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Interest Income		785,874	753,604
As Previously Reported [Member] | Commercial Real Estate [Member]
Segment Reporting Information [Line Items]
Net income		350,358	340,641
As Previously Reported [Member] | Commercial Real Estate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Interest Income		608,385	577,922
As Previously Reported [Member] | Discretionary Portfolio [Member]
Segment Reporting Information [Line Items]
Net income		163,538	59,272
As Previously Reported [Member] | Discretionary Portfolio [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Interest Income		345,926	97,626
As Previously Reported [Member] | Residential Mortgage Banking [Member]
Segment Reporting Information [Line Items]
Net income		79,678	88,578
As Previously Reported [Member] | Residential Mortgage Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Interest Income		70,655	63,939
As Previously Reported [Member] | Retail Banking [Member]
Segment Reporting Information [Line Items]
Net income		274,646	267,518
As Previously Reported [Member] | Retail Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Interest Income		1,074,125	917,041
As Previously Reported [Member] | All Other [Member]
Segment Reporting Information [Line Items]
Net income		(58,148)	(206,130)
As Previously Reported [Member] | All Other [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Interest Income		230,589	93,600
Impact of Changes [Member] | Business Banking [Member]
Segment Reporting Information [Line Items]
Net income		10,412	10,986
Impact of Changes [Member] | Business Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Interest Income		17,556	18,523
Impact of Changes [Member] | Commercial Banking [Member]
Segment Reporting Information [Line Items]
Net income		(317)	(144)
Impact of Changes [Member] | Commercial Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Interest Income		(535)	(242)
Impact of Changes [Member] | Residential Mortgage Banking [Member]
Segment Reporting Information [Line Items]
Net income		(24,225)	(14,216)
Impact of Changes [Member] | Residential Mortgage Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Interest Income		(40,846)	(23,970)
Impact of Changes [Member] | Retail Banking [Member]
Segment Reporting Information [Line Items]
Net income		19,727	20,579
Impact of Changes [Member] | Retail Banking [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Interest Income		33,263	34,699
Impact of Changes [Member] | All Other [Member]
Segment Reporting Information [Line Items]
Net income		(5,597)	(17,205)
Impact of Changes [Member] | All Other [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net Interest Income		$ (9,438)	$ (29,010)