Fair Value of Financial Assets and Liabilities - Derivative Instruments (Change in Fair Value of Series B Convertible Stock) (Details)	6 Months Ended
	Jun. 30, 2015 USD ($)
Derivatives And Fair Value [Line Items]
Balance, Warrant Liability	$ 5,826,000
Balance, Series B Redeemable Preferred Stock	12,320,000
Balance, Warrant Liability	3,984,000
Balance, Series B Redeemable Preferred Stock	10,032,000
Series B redeemable convertible preferred stock [Member]
Derivatives And Fair Value [Line Items]
Balance, Series B Redeemable Preferred Stock	12,320,000
Conversions to common stock	(636,000)
Changes in estimated fair value	(1,652,000)
Balance, Series B Redeemable Preferred Stock	10,032,000
Warrant Liability [Member]
Derivatives And Fair Value [Line Items]
Balance, Warrant Liability	5,826,000
Issuance of warrants	4,210,000
Exercise of warrants	(676,000)
Amount reclassified to additional paid in capital related to amendment of warrants	(5,462,000)
Changes in estimated fair value	86,000
Balance, Warrant Liability	$ 3,984,000