Fair Value Measurements (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jul. 01, 2017	Jul. 02, 2016	Jul. 01, 2017	Jul. 02, 2016	Dec. 31, 2016
Assets:
Available for sale securities	$ 357,090,000		$ 357,090,000		$ 404,693,000
Liabilities:
Transfers between fair value levels	0	$ 0	0	$ 0
U.S. government agencies [Member]
Assets:
Available for sale securities	18,035,000		18,035,000		86,526,000
Corporate debt securities [Member]
Assets:
Available for sale securities	339,055,000		339,055,000		318,167,000
Fair Value, Measurements, Recurring [Member]
Assets:
Money market funds	161,762,000		161,762,000		50,125,000
Derivative assets	361,000		361,000		10,625,000
Total	555,130,000		555,130,000		537,562,000
Liabilities:
Derivative liabilities	13,869,000		13,869,000		3,780,000
Fair Value, Measurements, Recurring [Member] | U.S. government agencies [Member]
Assets:
Available for sale securities	39,961,000		39,961,000		86,526,000
Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member]
Assets:
Available for sale securities	353,046,000		353,046,000		390,286,000
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets:
Money market funds	161,762,000		161,762,000		50,125,000
Derivative assets	0		0		0
Total	161,762,000		161,762,000		50,125,000
Liabilities:
Derivative liabilities	0		0		0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | U.S. government agencies [Member]
Assets:
Available for sale securities	0		0		0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Corporate debt securities [Member]
Assets:
Available for sale securities	0		0		0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Assets:
Money market funds	0		0		0
Derivative assets	361,000		361,000		10,625,000
Total	393,368,000		393,368,000		487,437,000
Liabilities:
Derivative liabilities	13,869,000		13,869,000		3,780,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. government agencies [Member]
Assets:
Available for sale securities	39,961,000		39,961,000		86,526,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Corporate debt securities [Member]
Assets:
Available for sale securities	353,046,000		353,046,000		390,286,000
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Assets:
Total	$ 0		$ 0		$ 0