Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets:
Available for sale securities	$ 723,449	$ 679,300
Corporate debt securities [Member]
Assets:
Available for sale securities	228,953	291,582
U.S. government agencies [Member]
Assets:
Available for sale securities	72,840	79,624
Fair Value, Measurements, Recurring [Member]
Assets:
Money market funds	273,546	193,066
Derivative assets	623
Total	575,962	564,272
Liabilities:
Derivative liabilities	549	2,138
Stock warrant liability	410	208
Total liabilities	959	2,346
Fair Value, Measurements, Recurring [Member] \| Corporate debt securities [Member]
Assets:
Available for sale securities	228,953	291,582
Fair Value, Measurements, Recurring [Member] \| U.S. government agencies [Member]
Assets:
Available for sale securities	72,840	79,624
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member]
Assets:
Money market funds	273,546	193,066
Derivative assets	0
Total	273,546	193,066
Liabilities:
Derivative liabilities	0	0
Stock warrant liability	0	0
Total liabilities	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 1 [Member] \| U.S. government agencies [Member]
Assets:
Available for sale securities	0	0
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member]
Assets:
Money market funds	0	0
Derivative assets	623
Total	302,416	371,206
Liabilities:
Derivative liabilities	549	2,138
Stock warrant liability	0	0
Total liabilities	549	2,138
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Corporate debt securities [Member]
Assets:
Available for sale securities	228,953	291,582
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| U.S. government agencies [Member]
Assets:
Available for sale securities	72,840	79,624
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	410	208
Total liabilities	410	208
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Corporate debt securities [Member]
Assets:
Available for sale securities	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| U.S. government agencies [Member]
Assets:
Available for sale securities	$ 0	$ 0

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 investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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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, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value of financial and nonfinancial obligations.

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Fair value portion of shares that an entity is required to redeem for cash or other assets at a fixed or determinable date or upon the occurrence of an event.

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