Investments - Carrying Value and Amortized Cost of Company's Investments Summarized by Major Security Type (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Estimated Fair Value	$ 316,767	$ 338,762
Debt and equity securities, Amortized Cost	316,573	339,145
Debt and equity securities, Unrealized Gains	293	39
Debt and equity securities, Unrealized Losses	(99)	(422)
Debt and equity securities, Estimated Fair Value	316,767	338,762
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized Cost	177,361	187,371
Debt securities, Unrealized Gains	178	39
Debt securities, Unrealized Losses	(92)	(344)
Debt securities, Estimated Fair Value	177,447	187,066
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized Cost	139,212	151,774
Debt securities, Unrealized Gains	115
Debt securities, Unrealized Losses	(7)	(78)
Debt securities, Estimated Fair Value	$ 139,320	$ 151,696

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Debt securities and equity securities fv ni amortized cost.

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Debt securities and equity securities fv ni unrealized gains.

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Debt securities and equity securities fv ni unrealized losses.

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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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-Publisher FASB
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-Topic 320
-SubTopic 10
-Section 50
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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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-Publisher FASB
-Name Accounting Standards Codification
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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), classified as current.

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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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