Summary of Significant Accounting Policies - Additional Information (Details) $ in Thousands	12 Months Ended			36 Months Ended
	Dec. 31, 2016 USD ($) segment shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($) shares	Dec. 31, 2016 USD ($)
Accounting Policies
Held-to-maturity investments	$ 0	$ 0		$ 0
Realized gains or losses from available-for-sale securities	0	0	$ 0
Losses from investments	0	0	0	0
Total restricted cash	$ 300	300		300
Number of operating segments | segment	1
Stock-based compensation	$ 6,847	$ 5,442	4,322
Accelerated amortization expense associated to employee's option			$ 1,300
Options to purchase common stock granted to employees and directors | shares	3,346,250	3,533,750	8,232,424
Fair value of options, vested	$ 6,900	$ 5,400	$ 4,200
Stock option exercises, Shares | shares	0	406,000	374,000
Intrinsic value of options exercised		$ 800	$ 600
Unrecognized compensation cost related to nonvested stock-based compensation	$ 13,400			13,400
Period for recognition of cost related to nonvested stock-based compensation arrangements	2 years 4 months 24 days
Prepaid expenses research and development	$ 1,400	$ 2,300		$ 1,400
Employees
Accounting Policies
Stock options, vesting period	4 years
Directors
Accounting Policies
Stock options, vesting period	3 years
Retiring Director's Stock Options
Accounting Policies
Stock-based compensation	$ 300
Minimum
Accounting Policies
Equipment depreciation period	3 years
Maximum
Accounting Policies
Equipment depreciation period	5 years