Segment information - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Segment Reporting [Abstract]
Taxable-equivalent adjustment	$ 6,217,000	$ 6,645,000	$ 12,667,000	$ 13,350,000