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Segment information - Additional Information (Detail) (USD $)
3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Operating Statistics [Line Items]        
Taxable equivalent adjustment $ 6,534,000 $ 6,546,000 $ 19,884,000 $ 19,341,000