Fair Value Measurements (Details) - USD ($)	3 Months Ended		6 Months Ended	12 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Dec. 31, 2017
Assets:
Available for sale securities	$ 252,347,000		$ 252,347,000	$ 337,334,000
Liabilities:
Transfers between fair value levels	0	$ 0	0
U.S. government agencies [Member]
Assets:
Available for sale securities	64,521,000		64,521,000	73,029,000
Corporate debt securities [Member]
Assets:
Available for sale securities	187,826,000		187,826,000	264,305,000
Level 3 [Member]
Assets:
Total	0		0	0
Other Assets [Member]
Liabilities:
Cash paid for acquisition				6,000,000
Fair Value, Measurements, Recurring [Member]
Assets:
Money market funds	153,241,000		153,241,000	193,066,000
Derivative assets	5,464,000		5,464,000
Total	451,408,000		451,408,000	564,272,000
Liabilities:
Derivative liabilities	736,000		736,000	2,138,000
Stock warrant liability	486,000		486,000	208,000
Total	1,222,000		1,222,000	2,346,000
Fair Value, Measurements, Recurring [Member] | U.S. government agencies [Member]
Assets:
Available for sale securities	74,897,000		74,897,000	79,624,000
Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member]
Assets:
Available for sale securities	217,806,000		217,806,000	291,582,000
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets:
Money market funds	153,241,000		153,241,000	193,066,000
Derivative assets	0		0
Total	153,241,000		153,241,000	193,066,000
Liabilities:
Derivative liabilities	0		0	0
Stock warrant liability	0		0	0
Total	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	5,464,000		5,464,000
Total	298,167,000		298,167,000	371,206,000
Liabilities:
Derivative liabilities	736,000		736,000	2,138,000
Stock warrant liability	0		0	0
Total	736,000		736,000	2,138,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. government agencies [Member]
Assets:
Available for sale securities	74,897,000		74,897,000	79,624,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Corporate debt securities [Member]
Assets:
Available for sale securities	217,806,000		217,806,000	291,582,000
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Assets:
Money market funds	0		0	0
Derivative assets	0		0
Total	0		0	0
Liabilities:
Derivative liabilities	0		0	0
Stock warrant liability	486,000		486,000	208,000
Total	486,000		486,000	208,000
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | U.S. government agencies [Member]
Assets:
Available for sale securities	0		0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Corporate debt securities [Member]
Assets:
Available for sale securities	$ 0		$ 0	$ 0