Net Loss per Share - Basic and Diluted Shares Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 26, 2015	Jun. 27, 2015	Mar. 28, 2015	Dec. 31, 2014	Sep. 27, 2014	Jun. 28, 2014	Mar. 29, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Net loss	$ (9,546)	$ 922	$ (5,779)	$ (11,930)	$ (2,988)	$ (3,848)	$ (3,951)	$ (10,027)	$ (26,333)	$ (20,814)	$ (17,265)
Denominator:
Weighted-average common shares outstanding									51,489	50,808	49,419
Basic and diluted net loss per common share (in dollars per share)									$ (0.51)	$ (0.41)	$ (0.35)
Potentially dilutive shares, weighted-average									6,120	5,020	5,308