Fair Value Measurements (Summary of Valuation of Company's Financial Instruments by Various Levels) (Details) - USD ($) $ in Thousands		Jul. 01, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		$ 147,955	$ 141,331
Total liabilities measured at fair value		507	8,192
Quoted market prices in active markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		144,131	139,532
Total liabilities measured at fair value		0	0
Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		3,824	1,799
Total liabilities measured at fair value		507	8,192
Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		0	0
Total liabilities measured at fair value		0	0
Foreign currency forward contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[1]	3,824	1,799
Total liabilities measured at fair value	[2]	507	8,192
Foreign currency forward contracts | Quoted market prices in active markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[1]	0	0
Total liabilities measured at fair value	[2]	0	0
Foreign currency forward contracts | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[1]	3,824	1,799
Total liabilities measured at fair value	[2]	507	8,192
Foreign currency forward contracts | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[1]	0	0
Total liabilities measured at fair value	[2]	0	0
Money-market funds | Cash equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		15,890	12,606
Money-market funds | Cash equivalents | Quoted market prices in active markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		15,890	12,606
Money-market funds | Cash equivalents | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		0	0
Money-market funds | Cash equivalents | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		0	0
U.S. treasuries | Available-for-sale securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[3]	125,625	124,670
U.S. treasuries | Available-for-sale securities | Quoted market prices in active markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[3]	125,625	124,670
U.S. treasuries | Available-for-sale securities | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[3]	0	0
U.S. treasuries | Available-for-sale securities | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[3]	0	0
Certificates of deposit | Available-for-sale securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[3]	154	162
Certificates of deposit | Available-for-sale securities | Quoted market prices in active markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[3]	154	162
Certificates of deposit | Available-for-sale securities | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[3]	0	0
Certificates of deposit | Available-for-sale securities | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[3]	0	0
Mutual funds | Trading securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[3]	2,462	2,094
Mutual funds | Trading securities | Quoted market prices in active markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[3]	2,462	2,094
Mutual funds | Trading securities | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[3]	0	0
Mutual funds | Trading securities | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[3]	$ 0	$ 0

[1] Included in Prepaid expenses and other current assets on the Company’s unaudited condensed consolidated balance sheets.

[2] Included in Other accrued liabilities on the Company’s unaudited condensed consolidated balance sheets.

[3] Included in Short-term investments on the Company’s unaudited condensed consolidated balance sheets.