Cash Equivalents and Investments - Amortized Cost and Fair Value of Cash Equivalents and Available for Sale Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Cash And Cash Equivalents [Line Items]
Amortized Cost	$ 27,624	$ 125,206
Amortized Cost	210,272
Unrealized Gains	0
Unrealized Losses	(126)
Fair Value	210,146
Cash Equivalents [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost		111,501
Unrealized Gains		0
Unrealized Losses		0
Fair Value		111,501
Short-term Investments [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost		143,895
Unrealized Gains		0
Unrealized Losses		(210)
Fair Value		143,685
Long-term Investments [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost		16,538
Unrealized Gains		0
Unrealized Losses		(20)
Fair Value		$ 16,518
Money Market Funds [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost	26,353
Unrealized Gains	0
Unrealized Losses	0
Fair Value	26,353
U.S. Treasury Securities [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost	79,489
Unrealized Gains	0
Unrealized Losses	(97)
Fair Value	79,392
Agency Bonds [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost	27,675
Unrealized Gains	0
Unrealized Losses	(9)
Fair Value	27,666
Commercial Paper [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost	62,872
Unrealized Gains	0
Unrealized Losses	(16)
Fair Value	62,856
Corporate Obligations [Member]
Cash And Cash Equivalents [Line Items]
Amortized Cost	13,883
Unrealized Gains	0
Unrealized Losses	(4)
Fair Value	$ 13,879

Cash and cash equivalents and available for sale debt securities accumulated gross unrealized gain before tax.

No definition available.

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- Definition

Cash and cash equivalents and available for sale debt securities accumulated gross unrealized loss before tax.

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- Definition

Cash and cash equivalents and available for sale debt securities amortized cost basis.

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- Definition

Cash and cash equivalents and available for sale securities debt securities.

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- Definition

Cash and cash equivalents unrealized gains.

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Cash and cash equivalents unrealized losses.

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- Definition

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on 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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Amortized cost 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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- Definition

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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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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