Fair Value Measurements - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 108,391	$ 81,430
Unrealized Gains		67
Unrealized Losses	(143)	(11)
Fair Value	108,248	81,486
Commercial Paper [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	36,567	30,125
Unrealized Gains		61
Unrealized Losses	(40)
Fair Value	36,527	30,186
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	71,824	51,305
Unrealized Gains		6
Unrealized Losses	(103)	(11)
Fair Value	$ 71,721	$ 51,300

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

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This item represents the cost of debt 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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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Amount of debt securities categorized neither as held-to-maturity nor trading.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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