Investments (Details) - USD ($) $ in Thousands	Apr. 01, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	$ 367,133	$ 316,086
Gross Unrealized Gains	7	3
Gross Unrealized Losses	(371)	(546)
Fair Value	366,769	315,543
Cash equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	148,056	34,587
Fair Value	148,056	34,587
Cash equivalents [Member] | Money market funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	94,689	23,110
Fair Value	94,689	23,110
Cash equivalents [Member] | Time deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	8,067	11,477
Fair Value	8,067	11,477
Cash equivalents [Member] | Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	44,800
Fair Value	44,800
Cash equivalents [Member] | Government and corporate bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	500
Fair Value	500
Short-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	153,605	185,693
Gross Unrealized Gains	7	3
Gross Unrealized Losses	(154)	(108)
Fair Value	153,458	185,588
Short-term investments [Member] | Time deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	29,538	40,639
Fair Value	29,538	40,639
Short-term investments [Member] | Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	23,175	22,325
Gross Unrealized Losses	(35)	(24)
Fair Value	23,140	22,301
Short-term investments [Member] | Government and corporate bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	100,892	122,729
Gross Unrealized Gains	7	3
Gross Unrealized Losses	(119)	(84)
Fair Value	100,780	122,648
Long-term investments [Member] | Government and corporate bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	65,472	95,806
Gross Unrealized Losses	(217)	(438)
Fair Value	$ 65,255	$ 95,368