Cash Equivalents and Investments - Available for Sale securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Money market funds, amortized cost	$ 78,527	$ 10,206	$ 6,029	$ 4,026
Amortized cost	176,470	61,009
Unrealized gains	0	0
Unrealized losses	(202)	(51)
Fair value	176,268	60,958
Corporate notes
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost	103,764	46,133
Available-for-sale securities, unrealized gains	0	0
Available-for-sale securities, unrealized losses	(184)	(35)
Available-for-sale securities, fair value	103,580	46,098
Government notes
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost	7,937	6,191
Available-for-sale securities, unrealized gains	0	0
Available-for-sale securities, unrealized losses	(18)	(16)
Available-for-sale securities, fair value	7,919	6,175
Money market funds
Schedule of Available-for-sale Securities [Line Items]
Money market funds, amortized cost	64,769	8,685
Money market funds, unrealized gains	0	0
Money market funds, unrealized losses	0	0
Money market funds, fair value	$ 64,769	$ 8,685

Cash, Cash Equivalents and Available-for-sale Securities

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Cash and Cash Equivalents, Accumulated Gross Unrealized Gain, before Tax

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Cash and Cash Equivalents, Accumulated Gross Unrealized Loss, before Tax

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Cash and Cash Equivalents and Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax

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Cash and Cash Equivalents and Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax

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Cash and Cash Equivalents and Available-for-sale Securities, Amortized Cost Basis

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Amount 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. Excludes cash and cash equivalents within disposal group and discontinued operation.

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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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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