Fair Value Measurements (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Oct. 01, 2016	Sep. 30, 2017	Oct. 01, 2016	Dec. 31, 2016
Assets:
Available for sale securities	$ 378,548,000		$ 378,548,000		$ 404,693,000
Liabilities:
Transfers between fair value levels	0	$ 0	0	$ 0
U.S. government agencies [Member]
Assets:
Available for sale securities	24,912,000		24,912,000		86,526,000
Corporate debt securities [Member]
Assets:
Available for sale securities	353,636,000		353,636,000		318,167,000
Fair Value, Measurements, Recurring [Member]
Assets:
Money market funds	129,790,000		129,790,000		50,125,000
Derivative assets	1,307,000		1,307,000		10,625,000
Total	557,695,000		557,695,000		537,562,000
Liabilities:
Derivative liabilities	11,826,000		11,826,000		3,780,000
Fair Value, Measurements, Recurring [Member] | U.S. government agencies [Member]
Assets:
Available for sale securities	50,707,000		50,707,000		86,526,000
Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member]
Assets:
Available for sale securities	375,891,000		375,891,000		390,286,000
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets:
Money market funds	129,790,000		129,790,000		50,125,000
Derivative assets	0		0		0
Total	129,790,000		129,790,000		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	1,307,000		1,307,000		10,625,000
Total	427,905,000		427,905,000		487,437,000
Liabilities:
Derivative liabilities	11,826,000		11,826,000		3,780,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. government agencies [Member]
Assets:
Available for sale securities	50,707,000		50,707,000		86,526,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Corporate debt securities [Member]
Assets:
Available for sale securities	375,891,000		375,891,000		390,286,000
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Assets:
Total	$ 0		$ 0		$ 0