Goodwill and Other Intangible Assets - Summary of Goodwill Assigned to Reportable Segments for Purposes of Testing for Impairment (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Nov. 01, 2015	Apr. 30, 2015	Dec. 31, 2014
Goodwill [Line Items]
Goodwill	$ 4,593,112			$ 3,524,625
Business Banking [Member]
Goodwill [Line Items]
Goodwill	864,366			748,907
Commercial Banking [Member]
Goodwill [Line Items]
Goodwill	1,401,873			907,524
Commercial Real Estate [Member]
Goodwill [Line Items]
Goodwill	654,389			349,197
Retail Banking [Member]
Goodwill [Line Items]
Goodwill	1,309,191			1,144,404
All Other [Member]
Goodwill [Line Items]
Goodwill	363,293			$ 374,593
All Other [Member] | Trade Processing Business Divestiture [Member]
Goodwill [Line Items]
Goodwill	(11,300)		$ (11,300)
Hudson City [Member]
Goodwill [Line Items]
Goodwill	1,068,487	$ 1,079,787
Hudson City [Member] | Business Banking [Member]
Goodwill [Line Items]
Goodwill	115,459
Hudson City [Member] | Commercial Banking [Member]
Goodwill [Line Items]
Goodwill	494,349
Hudson City [Member] | Commercial Real Estate [Member]
Goodwill [Line Items]
Goodwill	305,192
Hudson City [Member] | Retail Banking [Member]
Goodwill [Line Items]
Goodwill	$ 164,787