Segment information - Summary of Segment Information (Parenthetical) (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting [Abstract]
Taxable-equivalent adjustment	$ 6,105,000	$ 6,534,000	$ 18,772,000	$ 19,884,000