Concentration of Risks - Additional Information (Detail)	3 Months Ended		9 Months Ended			12 Months Ended
	Sep. 30, 2013 Total revenue [Member]	Sep. 30, 2012 Total revenue [Member]	Sep. 30, 2013 Total revenue [Member]	Sep. 30, 2012 Total revenue [Member]	Sep. 30, 2013 Total accounts receivable [Member]	Dec. 31, 2012 Total accounts receivable [Member]
Product Information [Line Items]
Number of customers represented more than 10%	0	1	0	0	2	0