Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)
0 Months Ended 3 Months Ended 12 Months Ended
Jun. 08, 2012
Jun. 30, 2012
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]          
Owned subsidiary     59.20%    
Share in Hampshire First Bank     43.00%    
Pre-tax gain $ 4,800,000 $ 4,800,000    $ 4,819,000   
Cash received from pre-tax gain 6,600,000     6,600,000  
Shares of NBTB 547,481        
Proceeds from sale of shares 11,100,000        
Total revenue     10.00%    
Cash on deposit     58,200,000    
Impairment charges     0 0  
Loans accrual of interest is discontinued at the time the loan     90 days    
Maximum loan value ratio     80.00%    
Originate loans with loan-to-value ratios     95.00%    
First time home buyers, loans with loan-to-value ratios     100.00%    
Period of repayment of loan     12 months 24 months  
Uncertain tax positions     0 0  
Interest and penalties     0 0 0
Interest or penalties     0 0 0
Unrecognized prior service cost     $ 28,000,000 $ 17,000,000  
Stock Option [Member]
         
Schedule of Available-for-sale Securities [Line Items]          
Options included in the computation of diluted earnings per share     13,971 17,500 58,600