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