Fair Value of Assets and Liabilities - Securities (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Jun. 28, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities - mutual funds	$ 8,296	$ 7,351
Derivative contracts, net	2,775	(2,739)
Assets, fair value	136,390	144,814
U.S. Government-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale marketable securities	27,596	17,608
Agency bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale marketable securities	56,605	51,951
Commercial paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale marketable securities	0	13,500
Certificates of deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale marketable securities	19,763	19,122
Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale marketable securities	10,625	27,119
Corporate Bond Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale marketable securities	10,730	10,902
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities - mutual funds	8,296	7,351
Derivative contracts, net	0	0
Assets, fair value	35,892	24,959
Fair Value, Inputs, Level 1 [Member] | U.S. Government-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale marketable securities	27,596	17,608
Fair Value, Inputs, Level 1 [Member] | Agency bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale marketable securities	0	0
Fair Value, Inputs, Level 1 [Member] | Commercial paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale marketable securities	0	0
Fair Value, Inputs, Level 1 [Member] | Certificates of deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale marketable securities	0	0
Fair Value, Inputs, Level 1 [Member] | Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale marketable securities	0	0
Fair Value, Inputs, Level 1 [Member] | Corporate Bond Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale marketable securities	0	0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities - mutual funds	0	0
Derivative contracts, net	2,775	(2,739)
Assets, fair value	100,498	119,855
Fair Value, Inputs, Level 2 [Member] | U.S. Government-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale marketable securities	0	0
Fair Value, Inputs, Level 2 [Member] | Agency bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale marketable securities	56,605	51,951
Fair Value, Inputs, Level 2 [Member] | Commercial paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale marketable securities	0	13,500
Fair Value, Inputs, Level 2 [Member] | Certificates of deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale marketable securities	19,763	19,122
Fair Value, Inputs, Level 2 [Member] | Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale marketable securities	10,625	27,119
Fair Value, Inputs, Level 2 [Member] | Corporate Bond Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale marketable securities	10,730	10,902
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities - mutual funds	0	0
Derivative contracts, net	0	0
Assets, fair value	0	0
Fair Value, Inputs, Level 3 [Member] | U.S. Government-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale marketable securities	0	0
Fair Value, Inputs, Level 3 [Member] | Agency bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale marketable securities	0	0
Fair Value, Inputs, Level 3 [Member] | Commercial paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale marketable securities	0	0
Fair Value, Inputs, Level 3 [Member] | Certificates of deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale marketable securities	0	0
Fair Value, Inputs, Level 3 [Member] | Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale marketable securities	0	0
Fair Value, Inputs, Level 3 [Member] | Corporate Bond Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale marketable securities	$ 0	$ 0