Investments Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Apr. 01, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
In Loss Position for Less Than 12 months, Fair Value	$ 144,428	$ 139,582
In Loss Position for Less Than 12 Months, Gross Unrealized Losses	350	332
Total Fair Value	144,428	139,582
Total Gross Unrealized Losses	350	332
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
In Loss Position for Less Than 12 months, Fair Value	14,272	17,693
In Loss Position for Less Than 12 Months, Gross Unrealized Losses	25	60
Total Fair Value	14,272	17,693
Total Gross Unrealized Losses	25	60
U.S. treasury bills and notes
Schedule of Available-for-sale Securities [Line Items]
In Loss Position for Less Than 12 months, Fair Value	10,477	18,463
In Loss Position for Less Than 12 Months, Gross Unrealized Losses	8	7
Total Fair Value	10,477	18,463
Total Gross Unrealized Losses	8	7
Securities and obligations of U.S. government agencies
Schedule of Available-for-sale Securities [Line Items]
In Loss Position for Less Than 12 months, Fair Value	119,679	103,426
In Loss Position for Less Than 12 Months, Gross Unrealized Losses	317	265
Total Fair Value	119,679	103,426
Total Gross Unrealized Losses	$ 317	$ 265