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Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair value, measurements, recurring - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 30, 2017
Assets    
Total assets $ 60,567 $ 206,969
Foreign currency exchange forward contracts    
Liabilities    
Foreign currency exchange forward contracts (167) (204)
Money market funds    
Assets    
Total assets 231 20,371
Certificates of deposit    
Assets    
Total assets 0 240
Commercial paper    
Assets    
Total assets 0 26,912
Corporate bonds    
Assets    
Total assets 46,881 118,558
U.S. agency notes    
Assets    
Total assets 5,485 5,480
U.S. treasuries    
Assets    
Total assets 7,970 35,408
Level 1    
Assets    
Total assets 8,201 55,779
Level 1 | Foreign currency exchange forward contracts    
Liabilities    
Foreign currency exchange forward contracts 0 0
Level 1 | Money market funds    
Assets    
Total assets 231 20,371
Level 1 | Certificates of deposit    
Assets    
Total assets 0 0
Level 1 | Commercial paper    
Assets    
Total assets 0 0
Level 1 | Corporate bonds    
Assets    
Total assets 0 0
Level 1 | U.S. agency notes    
Assets    
Total assets 0 0
Level 1 | U.S. treasuries    
Assets    
Total assets 7,970 35,408
Level 2    
Assets    
Total assets 52,366 151,190
Level 2 | Foreign currency exchange forward contracts    
Liabilities    
Foreign currency exchange forward contracts (167) (204)
Level 2 | Money market funds    
Assets    
Total assets 0 0
Level 2 | Certificates of deposit    
Assets    
Total assets 0 240
Level 2 | Commercial paper    
Assets    
Total assets 0 26,912
Level 2 | Corporate bonds    
Assets    
Total assets 46,881 118,558
Level 2 | U.S. agency notes    
Assets    
Total assets 5,485 5,480
Level 2 | U.S. treasuries    
Assets    
Total assets $ 0 $ 0