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Basis of Presentation (Details) (USD $)
In Millions, except Share data, unless otherwise specified
 0 Months Ended 9 Months Ended 12 Months Ended
Jan. 24, 2013
Sep. 30, 2013
Dec. 31, 2012
Basis Of Presentation [Line Items]      
Conversion costs   $ 11.5  
Common stock sold in the subscription and community offerings 35,558,927    
Common stock sold in the subscription and community offerings, price per share $ 10.00    
Common Stock Shares Outstanding 58,199,819 57,939,498 41,486,819
Demutualization, convertion rate   1.4029 1.4029
Northfield Bank Employee Stock Ownership Plan [Member]
      
Basis Of Presentation [Line Items]      
Common stock sold in the subscription and community offerings 1,422,357