Summary of Assumptions Used to Estimate Fair Value of Common Stock Warrant Liability (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Equity [Abstract]
Expected term	5 years 2 months 12 days	7 years 7 months 6 days	5 years 10 months 24 days	7 years 8 months 12 days
Volatility	53.00%	49.00%	52.00%	48.80%
Risk-free rate	1.47%	1.22%	1.18%	1.51%
Exercise multiple	2.8	2.8	2.8	2.8
Fair value of Signature common stock	$ 0.91	$ 0.37	$ 0.74	$ 0.31
Exercise price	$ 0.68	$ 0.68	$ 0.68	$ 0.68