Income Taxes - Current/Deferred (Details) - USD ($) $ in Thousands		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net operating loss carryforwards
U.S. federal corporate tax rate		35.00%
Federal AMT tax		$ 200
Current portion:
State		3	$ 3	$ 3
Foreign		7	42	83
Income tax provision, Current		10	45	86
Deferred portion:
Federal		(243)
Income tax provision		(233)	45	$ 86
Deferred tax assets:
Federal and state loss carryforwards		49,533	68,829
Reserves, accruals and other		391	519
Depreciation and amortization		1,100	1,718
Deferred stock-based compensation		2,483	4,287
Research and development credit carryforwards		15,487	13,867
Foreign tax and other credits		513	536
Total deferred tax assets		69,507	89,756
Deferred tax liabilities:
Acquired intangible assets and other		408	328
Less: Valuation allowance		(68,857)	(89,428)
Net deferred tax assets		242	0
Increase (Decrease) in valuation allowance during the year		20,600	$ 9,800
Forecast
Net operating loss carryforwards
U.S. federal corporate tax rate	21.00%
Federal
Net operating loss carryforwards
Net operating loss carryforwards, amount		196,100
State
Net operating loss carryforwards
Net operating loss carryforwards, amount		$ 119,600