Common Stock Warrant Liability (Tables)

12 Months Ended

Dec. 31, 2013

Changes in Fair Value of Common Stock Warrant Liability

The Company utilizes a trinomial lattice option pricing model to estimate the fair value of the common stock warrant liability. A decrease in the common stock warrant liability results in other income, while an increase in the common stock warrant liability results in other expense. The following table presents changes in the fair value of the common stock warrant liability during the years ended December 31, 2013 and 2012:

	Year Ended December 31,
(Dollars in thousands)	2013		2012
Beginning balance	$	2,350	$	1,403
Change in fair value of common stock warrant liability		6,950		947
Ending balance	$	9,300	$	2,350

Summary of Assumptions Used to Estimate Fair Value of Common Stock Warrant Liability

The following table summarizes the assumptions used to estimate the fair value of the common stock warrant liability as of December 31, 2013 and 2012:

	December 31,
	2013			2012
Expected term (years)		4.18			7.07
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