Fair Value of Financial Instruments, Level 3 Rollforward (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Fair value measurements of common stock warrants using significant unobservable inputs (level 3) [Roll Forward]
Balance at beginning of period	$ 1,258
Balance at end of period	820
Level 3 [Member]
Fair value measurements of common stock warrants using significant unobservable inputs (level 3) [Roll Forward]
Balance at beginning of period	1,258	$ 5,425
Exercise of warrants		(375)
Change in fair value of common stock warrant liability	(438)	(1,826)
Balance at end of period	$ 820	$ 3,224