Summary of Assumptions Used to Estimate Fair Value of Common Stock Warrant Liability (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Class Of Warrant Or Right [Line Items]
Expected term	4 years 1 month 6 days	7 years 2 months 12 days	5 years 3 months 18 days	7 years 7 months 6 days
Volatility	55.00%	51.00%	53.00%	49.50%
Risk-free rate	0.99%	1.07%	1.12%	1.36%
Exercise multiple	2.8	2.8	2.8	2.8
Market price of Signature common stock	$ 12.00	$ 4.60	$ 8.90	$ 3.60
Exercise price	$ 6.85	$ 6.85	$ 6.85	$ 6.85