Income Taxes - Additional Information (Details) - USD ($) $ in Thousands		12 Months Ended
	Jan. 01, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Valuation allowance		$ 86,021	$ 85,239
Increase (decrease) in valuation allowance due to deferred tax assets		800	13,400
Research and development tax credits		14,255	11,480
Unrecognized tax benefits, if recognized, would affect the effective tax rate		200	700	$ 700
Benefits from share compensation			$ 2,700
Provisional decrease to deferred tax assets		38,900
Provisional decrease in valuation allowance		41,400
Income tax benefit due to reduction in tax rate		2,500
Accounting Standards Update 2016-09 [Member]
Income Taxes [Line Items]
Increase (decrease) in valuation allowance due to deferred tax assets	$ 2,700
Federal [Member]
Income Taxes [Line Items]
Net operating loss carryforwards		$ 257,200
Net operating loss carryforwards expiration year		2018
Research and development tax credits		$ 11,000
Research and development tax credits beginning expiration year		2027
State [Member]
Income Taxes [Line Items]
Net operating loss carryforwards		$ 138,400
Research and development tax credits		4,100
Foreign [Member]
Income Taxes [Line Items]
Net operating loss carryforwards		$ 8,300
Net operating loss carryforwards expiration year		2022
Valuation Allowance, Operating Loss Carryforwards [Member]
Income Taxes [Line Items]
Increase (decrease) in valuation allowance due to deferred tax assets		$ (2,900)