Fair Value of Financial Assets and Liabilities (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Assets:
Marketable securities	$ 118,569	$ 188,652
Liabilities:
Transfers from Level 1 to Level 2	0	0
Transfers from Level 2 to Level 1	0	0
Transfers into or out of Level 3	0	0
Change in fair value of contingent consideration	866
Recurring
Assets:
Cash equivalents	44,904	19,339
Marketable securities	118,569	188,652
Total assets measured at fair value	163,473	207,991
Liabilities:
Acquisition-related contingent consideration	5,244	4,378
Total liabilities measured at fair value	5,244	4,378
Recurring \| Level 1
Assets:
Cash equivalents	33,722	19,339
Total assets measured at fair value	33,722	19,339
Recurring \| Level 2
Assets:
Cash equivalents	11,182
Marketable securities	118,569	188,652
Total assets measured at fair value	129,751	188,652
Recurring \| Level 3
Liabilities:
Acquisition-related contingent consideration	5,244	4,378
Total liabilities measured at fair value	$ 5,244	$ 4,378

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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
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Amount of increase (decrease) in the value of a contingent consideration liability, including, but not limited to, differences arising upon settlement.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
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- Definition

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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Fair value portion of arrangements with third parties, including, but not limited to, operating lease arrangement and arrangement in which the entity has agreed to expend funds to procure goods or services, or has agreed to commit resources to supply goods or services.

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Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

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Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

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Amount of transfer of financial instrument classified as an asset into (out of) level 3 of the fair value hierarchy.

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Fair value of financial and nonfinancial obligations.

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