Fair Value of Financial Instruments (Tables)

6 Months Ended

Jun. 30, 2014

Fair Value of Financial Instruments [Abstract]

Assets and liabilities measured at fair value

The tables below categorize assets and liabilities measured at fair value on a recurring basis for the periods presented:

	Fair Value		Fair value measurement using
	June 30, 2014		Level 1		Level 2		Level 3
Assets:
Cash and cash equivalents	$	65,557	$	65,557	$	–	$	–
Certificate of Deposit		325		325		–		–
Total Assets	$	65,882	$	65,882	$	–	$	–
Liabilities:
Common stock warrant liability	$	3,224	$	–	$	–	$	3,224

	Fair Value		Fair value measurement using
	December 31, 2013		Level 1		Level 2		Level 3
Assets:
Cash and cash equivalents	$	86,283	$	86,283	$	–	$	–
Certificate of Deposit		325		325		–		–
Total Assets	$	86,608	$	86,608	$	–	$	–
Liabilities:
Common stock warrant liability	$	5,425	$	–	$	–	$	5,425

Common stock warrants measured at Level 3 inputs on recurring basis

The table below summarizes the activity of Level 3 inputs measured on a recurring basis for the six months ended June 30, 2014 and 2013:

(in thousands)	Fair Value Measurements of Common Stock Warrants Using Significant Unobservable Inputs (Level 3)
Balance at December 31, 2013	$	5,425
Exercise of warrants		(375	)
Change in fair value of common stock warrant liability		(1,826	)
Balance at June 30, 2014	$	3,224

(in thousands)	Fair Value Measurements of Common Stock Warrants Using Significant Unobservable Inputs (Level 3)
Balance at December 31, 2012	$	6,305
Change in fair value of common stock warrant liability		(2,686	)
Balance at June 30, 2013	$	3,619

Significant unobservable input assumption used for valuation

The significant unobservable inputs used in the fair value measurement of the common stock warrants measured on a recurring basis are the historical volatility of our common stock market price, expected term of the applicable warrants, and the risk-free interest rate based on the U.S. Treasury yield curve in effect at the measurement date.  In addition to the significant unobservable inputs noted above, certain fair value measurements also take into account an assumption of the likelihood and timing of the occurrence of an event that would result in an adjustment to the exercise price in accordance with the anti-dilutive pricing provisions in the warrant.  Any significant increases or decreases in the unobservable inputs, with the exception of the risk-free interest rate, may result in significantly higher or lower fair value measurements.

Significant Unobservable Input Assumptions of Level 3 Valuations	June 30, 2014	December 31, 2013
Historical Volatility	52% – 61%	62% – 76%
Expected Term (in years)	0.7 – 1.6	0.4 – 2.1
Risk-free interest rate	0.08% – 0.34%	0.08% – 0.44%