Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Fair value of financial and nonfinancial obligations.

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The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

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Fair Value Measurements (Details) - USD ($)	3 Months Ended		12 Months Ended
Fair Value Measurements (Details) - USD ($)	Mar. 31, 2018	Apr. 01, 2017	Dec. 31, 2017
Assets:
Available for sale securities	$ 279,994,000		$ 337,334,000
Liabilities:
Transfers between fair value levels	0	$ 0
U.S. government agencies [Member]
Assets:
Available for sale securities	74,922,000		73,029,000
Corporate debt securities [Member]
Assets:
Available for sale securities	205,072,000		264,305,000
Fair Value, Measurements, Recurring [Member]
Assets:
Money market funds	169,430,000		193,066,000
Derivative assets	833,000
Total	541,283,000		564,272,000
Liabilities:
Derivative liabilities	430,000		2,138,000
Stock warrant liability	164,000		208,000
Total	594,000		2,346,000
Fair Value, Measurements, Recurring [Member] \| U.S. government agencies [Member]
Assets:
Available for sale securities	74,922,000		79,624,000
Fair Value, Measurements, Recurring [Member] \| Corporate debt securities [Member]
Assets:
Available for sale securities	296,098,000		291,582,000
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member]
Assets:
Money market funds	169,430,000		193,066,000
Derivative assets	0
Total	169,430,000		193,066,000
Liabilities:
Derivative liabilities	0		0
Stock warrant liability	0		0
Total	0		0
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| U.S. government agencies [Member]
Assets:
Available for sale securities	0		0
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Corporate debt securities [Member]
Assets:
Available for sale securities	0		0
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member]
Assets:
Money market funds	0		0
Derivative assets	833,000
Total	371,853,000		371,206,000
Liabilities:
Derivative liabilities	430,000		2,138,000
Stock warrant liability	0		0
Total	430,000		2,138,000
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| U.S. government agencies [Member]
Assets:
Available for sale securities	74,922,000		79,624,000
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Corporate debt securities [Member]
Assets:
Available for sale securities	296,098,000		291,582,000
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member]
Assets:
Money market funds	0		0
Derivative assets	0
Total	0		0
Liabilities:
Derivative liabilities	0		0
Stock warrant liability	164,000		208,000
Total	164,000		208,000
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| U.S. government agencies [Member]
Assets:
Available for sale securities	0		0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Corporate debt securities [Member]
Assets:
Available for sale securities	$ 0		0
Other Assets [Member]
Liabilities:
Cash paid for acquisition			$ 6,000,000