Stock Incentive Plans - Stock Option Plan Activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Stock options
Outstanding at beginning of year (in shares)	1,723,554
Granted (in shares)	1,068,845
Exercised (in shares)	(230,019)
Cancelled or forfeited (in shares)	(292,211)
Outstanding at end of year (in shares)	2,270,169	1,723,554
Exercisable at end of year (in shares)	851,769
Vested or expected to vest at end of year (in shares)	2,177,976
Weighted-Average Exercise Price
Outstanding at beginning of year (in dollars per share)	$ 6.66
Granted (in dollars per share)	4.49
Exercised (in dollars per share)	2.00
Cancelled or forfeited (in dollars per share)	6.60
Outstanding at end of year (in dollars per share)	6.11	$ 6.66
Exercisable at end of year (in dollars per share)	5.79
Vested or expected to vest at end of year (in dollars per share)	$ 6.12
Weighted Average Remaining Contractual Term (years)
Outstanding at end of year	8 years 1 month 21 days	7 years 11 months 1 day
Exercisable at end of year	6 years 7 months 21 days
Vested or expected to vest at end of year	8 years 1 month 2 days
Aggregate Intrinsic Value
Outstanding at the beginning of the year	$ 4,267,000.000
Outstanding at the end of the year	613,000.000	$ 4,267,000.000
Exercisable at the end of the year	607,000.000
Vested or expected to vest at end of year	613,000.000
Unrecognized stock-based compensation expense
Intrinsic value of options exercised	500,000	300,000	$ 600,000
Fair value of options vested	$ 2,300,000	$ 1,400,000	$ 500,000
Weighted average of the assumptions used to compute employee stock based compensation
Expected term (in years)	6 years 3 months
Expected dividend yield (as a percent)	0.00%	0.00%	0.00%
Options to purchase Common Stock
Unrecognized stock-based compensation expense
Unrecognized compensation expense related to unvested stock option awards	$ 4,900,000
Weighted-average amortization period over which cost is expected to be recognized	2 years 6 months
Minimum
Weighted average of the assumptions used to compute employee stock based compensation
Weighted-average expected volatility (as a percent)	72.00%	73.60%	75.20%
Expected term (in years)		6 years 2 months 1 day	6 years 3 months
Risk free interest rate (as a percent)	1.24%	0.92%	1.71%
Maximum
Weighted average of the assumptions used to compute employee stock based compensation
Weighted-average expected volatility (as a percent)	73.90%	86.80%	83.40%
Expected term (in years)		10 years	10 years
Risk free interest rate (as a percent)	2.00%	2.45%	3.01%