Activity of Exercisable Common Stock Options (Detail) (USD $)	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Number of Shares
Vested common stock options, beginning balance	1,813,252
Common stock options exercised	(66,667)	(85,334)
Common stock options vested	3,332,083	1,898,586
Vested common stock options, ending balance	5,078,668	1,813,252
Weighted Average Exercise Price Per Share
Vested common stock options, beginning balance	$ 0.52
Common stock options exercised	$ 0.30	$ 0.30
Common stock options vested	$ 0.55	$ 0.51
Vested common stock options, ending balance	$ 0.54	$ 0.52