Recent Accounting Pronouncements - Revenue From Contracts With Customers Beginning Balance Sheet Impact (Details) - USD ($) $ in Thousands	Sep. 28, 2018	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net	$ 77,986		$ 69,844
Prepaid expenses and other current assets	24,226		18,931
Other long-term assets	38,624		42,913
Deferred revenue	48,679		52,429
Other non-current liabilities	18,989		22,626
Accumulated deficit	(2,070,746)		$ (2,057,812)
Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net		$ 71,625
Prepaid expenses and other current assets		22,509
Other long-term assets		43,686
Deferred revenue		47,603
Other non-current liabilities		22,153
Accumulated deficit		(2,046,381)
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net	74,949
Prepaid expenses and other current assets	19,681
Other long-term assets	38,082
Deferred revenue	53,370
Other non-current liabilities	19,450
Accumulated deficit	(2,084,022)
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net	3,037	1,781
Prepaid expenses and other current assets	4,545	3,578
Other long-term assets	542	773
Deferred revenue	(4,691)	(4,826)
Other non-current liabilities	(461)	(473)
Accumulated deficit	$ 13,276	$ 11,431