Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets:
Securities	$ 81,750
Other assets	24,787	$ 6,294
Fair Value Measurements, Recurring Basis
Assets:
Cash and cash equivalents	78,200	146,200
Total assets measured at fair value	162,679	160,849
Other assets	2,700	1,900
Fair Value Measurements, Recurring Basis \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Total assets measured at fair value	53,158	46,257
Fair Value Measurements, Recurring Basis \| Significant Other Observable Inputs (Level 2)
Assets:
Total assets measured at fair value	106,813	112,702
Fair Value Measurements, Recurring Basis \| Significant Unobservable Inputs (Level 3)
Assets:
Total assets measured at fair value	2,708	1,890
Fair Value Measurements, Recurring Basis \| Money market funds
Assets:
Cash and cash equivalents	53,158	46,257
Fair Value Measurements, Recurring Basis \| Money market funds \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash and cash equivalents	53,158	46,257
Fair Value Measurements, Recurring Basis \| Money market funds \| Significant Other Observable Inputs (Level 2)
Assets:
Cash and cash equivalents	0	0
Fair Value Measurements, Recurring Basis \| Money market funds \| Significant Unobservable Inputs (Level 3)
Assets:
Cash and cash equivalents	0	0
Fair Value Measurements, Recurring Basis \| U.S. Treasury securities
Assets:
Securities		12,488
Fair Value Measurements, Recurring Basis \| U.S. Treasury securities \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Securities		0
Fair Value Measurements, Recurring Basis \| U.S. Treasury securities \| Significant Other Observable Inputs (Level 2)
Assets:
Securities		12,488
Fair Value Measurements, Recurring Basis \| U.S. Treasury securities \| Significant Unobservable Inputs (Level 3)
Assets:
Securities		0
Fair Value Measurements, Recurring Basis \| Government-sponsored enterprises
Assets:
Securities	17,389
Fair Value Measurements, Recurring Basis \| Government-sponsored enterprises \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Securities	0
Fair Value Measurements, Recurring Basis \| Government-sponsored enterprises \| Significant Other Observable Inputs (Level 2)
Assets:
Securities	17,389
Fair Value Measurements, Recurring Basis \| Government-sponsored enterprises \| Significant Unobservable Inputs (Level 3)
Assets:
Securities	0
Fair Value Measurements, Recurring Basis \| Corporate debt securities
Assets:
Securities	89,424	100,214
Fair Value Measurements, Recurring Basis \| Corporate debt securities \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Securities	0	0
Fair Value Measurements, Recurring Basis \| Corporate debt securities \| Significant Other Observable Inputs (Level 2)
Assets:
Securities	89,424	100,214
Fair Value Measurements, Recurring Basis \| Corporate debt securities \| Significant Unobservable Inputs (Level 3)
Assets:
Securities	0	0
Fair Value Measurements, Recurring Basis \| Common stock warrants
Assets:
Securities	2,708	1,890
Fair Value Measurements, Recurring Basis \| Common stock warrants \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Securities	0	0
Fair Value Measurements, Recurring Basis \| Common stock warrants \| Significant Other Observable Inputs (Level 2)
Assets:
Securities	0	0
Fair Value Measurements, Recurring Basis \| Common stock warrants \| Significant Unobservable Inputs (Level 3)
Assets:
Securities	$ 2,708	$ 1,890

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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-Publisher FASB
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Amount of noncurrent assets classified as other.

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