Cash Equivalents and Short-Term Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Money market funds at amortized cost	$ 72,454	$ 45,214	$ 112,921	$ 75,354
Total amortized cost	139,204	98,478
Gross Unrealized Gains	0	4
Gross Unrealized Losses	(49)	(28)
Money market funds, fair value	70,502	36,414
U.S. Treasury securities, fair value	68,653	62,040
Total fair value	139,155	98,454
Money market funds
Schedule of Available-for-sale Securities [Line Items]
Money market funds at amortized cost	70,502	36,412
Money market funds, Gross Unrealized Gains	0	2
Money market funds, fair value	70,502	36,414
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
U.S. Treasury securities, amortized cost	68,702	62,066
U.S. Treasury securities, Gross Unrealized Gains	0	2
Gross Unrealized Losses	(49)	(28)
U.S. Treasury securities, fair value	$ 68,653	$ 62,040

Cash And Cash Equivalents, Accumulated Gross Unrealized Gain, Before Tax

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- Definition

Cash And Cash Equivalents And Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, Before Tax

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- Definition

Cash, Cash Equivalents And Available-for-sale, Debt Securities

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Cash, Cash Equivalents And Available-for-sale Debt Securities, Amortized Cost

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- Definition

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

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-Publisher FASB
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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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