Share-based compensation - Additional Information (Detail) (USD $)	9 Months Ended			9 Months Ended				12 Months Ended	9 Months Ended		1 Months Ended		9 Months Ended	1 Months Ended		9 Months Ended	1 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 Maximum	Sep. 30, 2013 Minimum	Sep. 30, 2013 Stock Option Plan	Sep. 30, 2012 Stock Option Plan	Dec. 31, 2012 Stock Option Plan	Sep. 30, 2013 2004 and 2008 Stock Incentive Plan Maximum	Sep. 30, 2013 2004 and 2008 Stock Incentive Plan Minimum	Sep. 10, 2004 Stock Incentive Plan 2004	Sep. 10, 2004 Stock Incentive Plan 2004 Maximum	Sep. 30, 2013 Stock Incentive Plan 2004 Maximum	Aug. 27, 2008 2008 Stock Incentive Plan	Aug. 27, 2008 2008 Stock Incentive Plan Maximum	Sep. 30, 2013 2008 Stock Incentive Plan Maximum	Sep. 10, 2004 2004 Stock Incentive Plan	Aug. 19, 1999 1997 Employee Stock Option Plan	Oct. 31, 1994 1994 Employee Stock Option Plan
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share based compensation expense	$ 53,000	$ 60,000
Issuance of stock option to purchase stock	5,500,000
Common stock reserved for issuance	2,535,087													3,000,000			2,500,000	2,000,000	725,000
Share-based compensation arrangement by share based payment award options granted contractual term				10 years	5 years
Stock option awards vesting period				2 years
Options granted						110,000	30,843	30,843
Unrecognized compensation cost related to bon-vested	130,000
Plan expiration date														Aug. 27, 2008			Sep. 10, 2014	Jun. 11, 2007	Nov. 01, 2004
Options grant, expiration period, years, maximum											5 years	10 years		5 years	10 years
Intrinsic value of options exercised			$ 267,000
Share based compensation arrangement by share based payment award percentage of fair market Person holding more then 10% voting stock													110.00%			110.00%
Share based compensation arrangement by share based payment award options grants in period gross									50,000	10,000