Fair Value Measurments (Tables)

12 Months Ended

Dec. 31, 2015

Fair Value Measurements.

Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis

        The following table summarizes the financial instruments measured at fair value on a recurring basis in the consolidated balance sheets (in thousands):

	Total		Quoted Prices in Active Markets for Identical Items (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Other Unobservable Inputs (Level 3)
Balance at December 31, 2015
Common stock warrant liability	$	5,735	$	—	$	—	$	5,735
Balance at December 31, 2014
Common stock warrant liability	$	9,418	$	—	$	—	$	9,418

Assumptions used to calculate common stock warrants

	At December 31, 2015	At December 31, 2014
Risk-free interest rate	0.5% - 1.28%	0.21% - 1.35%
Volatility	79.82% - 128.35%	119.16% - 136.62%
Expected average term	0.42 - 3.04	1.42 - 4.04

Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation

        The following table shows reconciliations of the beginning and ending balances for financial instruments measured at fair value on a recurring basis using significant unobservable inputs (in thousands):

	Fair Value Measurement Using Significant Unobservable Inputs
Common stock warrant liability	2015			2014
Beginning of period	$	9,418		$	28,830
Change in fair value of common stock warrants		(3,661	)		52,260
Issuance of common stock warrants		—			11,773
Exercise of common stock warrants		(22	)		(83,445	)
End of period	$	5,735		$	9,418