Recent Accounting Pronouncements - Narratives (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 28, 2018	Sep. 28, 2018	Jan. 01, 2018	Dec. 31, 2017	Sep. 29, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated deficit	$ (2,070,746)	$ (2,070,746)		$ (2,057,812)
Net Capitalized Contract Costs	1,800	1,800
Capitalized Contract Cost, Amortization	400	900
Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated deficit			$ (2,046,381)
Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated deficit	13,276	13,276	11,431
Accounting Standards Update 2016-18 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Restricted Cash	200	200		$ 1,700	$ 2,000	$ 1,800
Included In Deferred Revenue Balance At January 1, 2018 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred Revenue, Revenue Recognized	7,800	43,200
Prepaid Expenses and Other Current Assets [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Capitalized Contract Costs	1,300	1,300
Other Noncurrent Assets [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Capitalized Contract Costs	$ 500	$ 500
Sales Commission Contract Costs [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated deficit			$ 1,400
Minimum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Capitalized Contract Cost, Amortization Period			1 year