Net Loss Per Common Share (Anti-Dilutive Common Share Equivalents) (Details) - $ / shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common share equivalents (in shares)	12,163,804	13,617,265
Warrants to purchase
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common share equivalents (in shares)	1,985,846	1,985,846
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common share equivalents (in shares)	8,234,689	10,056,752
Common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common share equivalents (in shares)	2,007,846
Common stock | Warrants to purchase
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common share equivalents (in shares)	22,000	22,000
Restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common share equivalents (in shares)	3,874,666	3,487,022
Employee stock purchase program
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common share equivalents (in shares)	32,449	51,491
Weighted Average
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Price of shares entitled to purchase (in dollars per share)	$ 10.70