Segment information - Information about Company's Segments (Parenthetical) (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Taxable-equivalent adjustment	$ 26,391,000	$ 25,876,000	$ 24,023,000