Summary of Assumptions Used to Estimate Fair Value of Common Stock Warrant Liability (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Class Of Warrant Or Right [Line Items]
Expected term (years)	4 years 2 months 5 days	7 years 26 days
Volatility	55.00%	51.00%
Risk-free rate	1.35%	1.19%
Exercise multiple	2.80	2.80
Market value of Signature common stock	$ 10.75	$ 4.10
Weighted average exercise price	$ 6.85	$ 6.85