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Goodwill and other intangible assets - Summary of Goodwill Assigned to Reportable Segments for Purposes of Testing for Impairment (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Goodwill [Line Items]    
Goodwill $ 3,524,625 $ 3,524,625
Business Banking [Member]
   
Goodwill [Line Items]    
Goodwill 748,907 748,907
Commercial Banking [Member]
   
Goodwill [Line Items]    
Goodwill 907,524 907,524
Commercial Real Estate [Member]
   
Goodwill [Line Items]    
Goodwill 349,197 349,197
Discretionary Portfolio [Member]
   
Goodwill [Line Items]    
Goodwill      
Residential Mortgage Banking [Member]
   
Goodwill [Line Items]    
Goodwill      
Retail Banking [Member]
   
Goodwill [Line Items]    
Goodwill 1,144,404 1,144,404
All Other [Member]
   
Goodwill [Line Items]    
Goodwill $ 374,593 $ 374,593